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• 1 2021-06-21T15:33:52-04:00 Graphs and Charts: Benefits and Challenges 2 Graphs are a common type of data visualization and can be distinguished from the other types of data visualizations by their well-defined reference system, such as horizontal or vertical axes on which data are plotted. Charts are topical or categorical data visualizations with no inherent reference systems, such as pie charts or word clouds. Graphs and charts can tell a powerful story and they can serve as compelling evidence. However, they also present multiple challenges for students trying to make sense of them. plain 2021-11-12T18:47:11-05:00 Graphs are a common type of data visualization in social studies and can be distinguished from the other types of data visualizations by their well-defined reference system, such as horizontal or vertical axes on which data are plotted. There are temporal graphs such as line graphs, and topical or categorical graphs such as bar graphs and scatter plots. Charts, also relatively common in social studies, are topical or categorical data visualizations with no inherent reference systems, such as pie charts or word clouds.[13]
  
  Although almost all states require that students study or use graphs or charts in social studies at some point in their school career, only 14 states require students to do so across elementary, middle, and high school. All too frequently, graphs and charts are not mentioned in social studies disciplines where you might expect them to have a prominent role. For example, twenty-two state standards documents contain no explicit references to graphs and charts in history, 31 state standards documents contain no explicit references in civics, and 30 contain no references in economics. Furthermore, it is strikingly uncommon for state standards to recommend that students learn how to critically analyze or evaluate graphs and charts.[14] 
  
  Why does this matter? How will students benefit from learning about graphs and charts in social studies? What challenges will students face when they attempt to learn from and with them?
  
  Graphs and Charts Can Tell a Powerful Story
  The benefits of learning to read graphs and charts in social studies are ample. First, graphs and charts can tell a powerful story. Consider using the following stacked and multi-set bar graph plus pie chart to teach students about the characteristics and consequences of World War II. What can students learn from this graph?The graph above not only provides a picture of the overall number of deaths, but also allows students to compare losses across countries. In addition, it breaks down losses by civilian and military deaths and provides a snapshot of the differences between the proportion of civilian and military deaths suffered by Axis and Allied powers. Due in large part to the Holocaust, the proportion of civilian Allied deaths among all deaths is striking. And look at the two bars associated with Poland. The sheer number of deaths does not stand out, but the bar representing the number of deaths as a percentage of the country's population tells a different story – Poland lost around 18% of its population, and most of those losses were Polish Jews.
  
  Now consider what students can learn from this interactive data documentary, The Fallen of World War II, created by Neil Halloran. Halloran creates a compelling narrative about the loss of human life during WWII. Using icons that each represent 1000 people – civilians or soldiers – Halloran uses categorical bar graphs to compare deaths across countries, as well as a temporal bar graph to show the progression of losses during the war. He also disaggregates data so that you can see the proportion of deaths by battles and fronts.  Moreover, Halloran connects to his mostly U.S. audience by beginning with numbers of U.S. deaths and comparing World War II losses with other wars of recent memory. In addition, he intersperses the data with photographic images, adding an important human element to the otherwise nameless and faceless statistics. While Halloran's data documentary represented a sophisticated use of technology to be sure, he nonetheless uses strategies – disaggregating data, connecting to people's lived experiences, and humanizing data with images and stories – that all teachers can use. 
  
  Graphs and Charts Can Serve as Powerful Evidence
  Graphs can also serve as compelling evidence for arguments. The article below from the Pew Research Center, for example, relies heavily on graphs as evidence for the argument that differences in political values – not gender, race or ethnicity, religion, or education – are what have led to increasing divisions between political parties and between the people who identify with different parties.  
  
  
  While the use of graphs to support arguments about current topics is not surprising, it is important to note that historians have become increasingly creative about using graphs and other data visualizations to support their arguments about the past. For example, in an article highlighting results from a digital history project on slave narratives, Lauren Tilton uses graphs to show how "dialect was not only racialized but also connected to a particular (cultural) geography—the American South." In another article, Shawn Martin uses multiple graphs to illustrate topical changes in American scientific journals from 1888 to 1922. 
  
  Challenges to Student Learning
  Although graphs and charts can be powerful tools for students in social studies, making sense of them can be challenging. Unfortunately, teachers will not find much guidance on how to teach students to read them or how to build students' graph reading skills over time in state standards documents. Among the 14 states that consistently address graphs and charts across school levels only 6 provide any guidance to teachers about how to build graph skills over grade and school levels. Michigan is one of them, but only if you pay attention to the "process and skills" standards that precede the content expectations at different school levels. For example, Michigan's Social Studies Process and Skills standards require that, by the end of elementary school, students express information in line and bar graphs, and use data from graphs or charts to answer questions. Then in middle school, Michigan Social Studies Standards recommend that students add pie charts to their repertoire and that they know how to evaluate data presented in graphs. This guidance, while better than in many states, is still insufficient.
  
  First, students are going to encounter more than just line graphs, bar graphs, and pie charts during their time in school, and they'll certainly encounter pie charts before middle school. In fact, a recent analysis of school textbooks reveals that elementary school students will encounter pie charts, bar graphs, line graphs, and even the occasional area graph or multi-set bar graph (i.e., bar graphs with multiple cases compared across one variable category). Plus, by middle school they'll encounter these same types of data visualizations, as well as population pyramids, stacked bar graphs, and multilayered graphical representations that, for example, combine bar graphs and line graphs. If students did not become adept at working with the multiple types of graphs and charts that they encountered in elementary school, the variety of middle school graphs and charts will be even more overwhelming. 
  
  Second, standards provide little to no guidance on what students need to do to be able to draw facts and inferences from graphs and charts. Yet, research has indicated that reading graphs is a complex process with several discrete steps, and a breakdown in any one of the steps could negatively impact a student's understanding.[15] 
  
  First, viewers must read the data. Because data is encoded as various visual elements (e.g., shapes, colors, text) in a graph, readers must identify the important visual elements such as the shape and directions of a line or numbers on an axis.[16] Ignoring or skipping over visual elements is similar to skipping over words or punctuation in a paragraph – doing so can change the meaning of the passage. When you look at the U.S. Census Bureau graph below, for example, what visual elements do you see?
  

  Click through this animation to see the important visual elements of this graph

  Then viewers must see between the data. They must relate the visual elements to the conceptual relations that are represented by those elements – that is, they must map between the elements themselves and their meaning, such as recognizing that a curved line implies an accelerating relationship. This ability is largely dependent upon the viewer’s experience with different graphics, or their understanding of graphical conventions.[17] What does each line on the multi-set graph above show you? What's the difference in what they're showing you? 
  
  Finally, a viewer must be able to make associations between the graphic representation and the context or the referents (e.g., immigrant population or number of casualties) that are being quantified.[18] That is, they must read beyond the data. This last factor implies that students should work with graphs within a specific context, rather than as abstractions disconnected from content.  Indeed, psychologists Priti Shah and James Hoeffner argued, “students taught about graph reading in an abstract context may not be able to apply graph reading knowledge to real contexts in which their beliefs or expectations might influence their interpretations." They further argued that an added benefit to teaching graphs in the context of the disciplines is that “students may also learn that graphs are a tool for critically evaluating data, not just a tool for information delivery." [19]
  
  Again, consider the graph above. What is the graph trying to tell you? And what can you actually infer from the graph? The space between those two questions is where even students who are good at reading graphs (say, from their learning in math class) may struggle. Drawing inferences combines evidence and reasoning, and reasoning often requires knowledge beyond what is right in front of us. For example, what is meant by the word poverty in this context? What does it mean to live in poverty? Is there a difference in the poverty rate among different groups of people? Does this graph better support a positive or negative argument about economic progress in the country? Can we rely on it for answering this question? These are the type of questions you want students to ask in order to read beyond the data and draw inferences, but it is not something they are apt to do on their own. As teachers we must therefore consider these questions on our own before presenting data visualizations to students, and be prepared to support students in asking such questions themselves. 
• 1 2021-06-21T15:33:49-04:00 Maps: Benefits and Challenges 1 Maps are essential tools in social studies. They provide important spatial context for understanding past and current events and patterns. As ubiquitous as maps are, they can be challenging to read for students, or worse, may enforce misconceived world views. plain 2021-06-21T15:33:49-04:00 Maps are geospatial data visualizations that use a latitude and longitude reference system overlaid with physical or political markers. [20]  Maps can also be overlaid with one or more layers of data that provide quantitative statistics, or information about connections, movement, spatial expansion, and more. As shown in the module, What kinds of data visualizations will students encounter in social studies?, maps can come in a variety of forms, including bubble maps, choropleth maps, connection maps, distribution maps, and flow maps. Maps are also often overlaid with data about change over time which transforms them into spatiotemporal visualizations, allowing viewers to simultaneously address “where” and “when” questions. 
  
  Maps as Tools for Spatial Reasoning about Past and Present
  Maps have a critical role to play across all the core social studies disciplines. In fact, they are the most common type of data visualization students will encounter in social studies texts. They help us determine geographic facts including absolute or relative location, the human and physical characteristics of places, as well as the physical connections between places. We can then use these geographic facts to employ spatial reasoning about the past or present, answering questions such as:

  How are/were places similar or different?
  How is this place likely to affect or influence another?
  How can this place be grouped with another?
  How do things change between places? How do we know when we’ve gone from one place to another?
  What larger places is this place inside? What smaller areas are inside of it? What impact does this hierarchy have on the places involved?
  What other places are like this place?
  Are things located in a distinctive arrangement? What patterns are evident?
  What features or phenomena tend to occur together in the same places?

  Maps can even help us understand ourselves—our own worldviews. Simple activities like the drawing and labeling of sketch maps can helps students reflect on what they know—and don't know—about the world. And GIS technologies allow students to track their own personal movement data, which they can use to examine where they go or don't go and how their everyday movements might shape their personal sense of place. 
  
  In history, maps are fundamental for telling and reading the stories of past. They allow historians to discover and visualize relationships hidden in written text and graphs, and conceive of space, place, and time in concert, and at both small and large scales. Maps make the invisible visible—revealing ways that people moved over long stretches of time, or how diseases or languages spread.  Maps can show both the world and parts of the world at the same time, and thus, make the shifting of scales a possibility for scholars who want to move between local, regional, national, and global frames. In short, maps are powerful tools for inquiry, as is evidenced by the widespread use of maps and GIS technologies in digital history projects over the last several years. Notable examples include Mapping Segregation in Washington, D.C., Digital Harlem, and Voting America.
  
  For learners, maps help establish a spatial context for events, interactions, and developments. To be sure, students will likely see them all over the texts they use to study history. Maps account for approximately 72% of all data visualizations in middle school world history textbooks, and 58% of all data visualizations in middle school U.S. history textbooks. Numbers are similar in high school history textbooks—maps account for 59% of all data visualizations in world history textbooks, and 49% in U.S history.  And the majority of these high school history maps are spatiotemporal, showing change or movement over both time and space.[21] As Clara Webb argues in the journal World History Connected, “Maps can be powerful visual tools that provoke us to think about the how and why of history.”[22]   
  
  While many of the maps students encounter in school are intended to serve as visual aids to the spatial dimensions of historical developments, they can also be primary sources that provide students additional insight into change over time, causation, and historical perspectives.  Indeed, maps as way-finding tools have played a significant role human lives throughout the world’s past, and have been as wide-ranging as an early map created by Egyptians of the 12th century BCE to find their way to gold and silver, to the iconic 1930s map of the London Underground. Some way-finding devices proved truly transformative in history, such as the map that may have helped encourage Christopher Columbus to find his way to the East Indies with a westward route, charting the course to the great global convergence of the Eastern and Western worlds. Other maps—such as those of the Chinese in the classical age, or the British at the height of imperialism—reveal to the historian the habits, thoughts, and perspectives of the people who created them, helping us understand how people of the past saw the world, where they had gone, and where they thought they could go.Approaches to Map Reading and the Challenges Students Face
  Because someone within a particular context and with a particular purpose created any given map, it’s important that students question the premises and choices that underlie maps, analyzing the arguments that the maps are conveying, and features that were included and left out in making these arguments. Maps have been and continue to be keys to knowledge and power, as well as mediums of propaganda.  
  
  It’s also important for students to recognize that all maps are inherently incomplete and distorted. They are merely representations of real space, and cannot possibly show us everything about the place they are intended to represent. Maps at different scales show different levels of detail but that does not necessarily make one more accurate than another. And of course, in the shape and/or size of landmasses. The Mercator projection, for example, which students probably see in their textbooks and on classroom walls most often, famously makes Africa look much smaller and Greenland look much bigger than they are in reality. As Tim Marshall points out in his book Prisoners of Geography, “You could fit the United States, Greenland, India, China, Spain, France, German, and the UK into Africa and still have room for most of Eastern Europe.”[23] Without revealing different projections and their distortions to students, there is no reason to think that they will ever understand, and they may then walk away from school with misconceptions about the world. 
  
  There are other challenges students face in reading maps as well. First, young elementary students may fail to recognize that maps play an important communicative purpose within a larger text, which may prevent them from carefully examining the map and trying to determine its meaning within the text, and then using it to facilitate their understanding of the geopolitical or social issue at hand.[24] Young children may have difficulty recognizing how maps are oriented in relation to real space, and they may have difficulty interpreting the abstract symbol system on maps (e.g., grid system, colors, icons), or recognizing how the symbol systems are used to communicate information about the real world. The concept of scale seems to be particularly difficult for students to learn because it is highly abstract and requires proportional reasoning, measuring abilities, and an understanding of the relationship between a map’s scale and the amount of detail that can be shown on the maps.[25] Furthermore, students must recognize the interconnectedness of map elements and exercise a degree of proportional reasoning. Researchers Bausmith and Leinhardt have thus argued that teachers should provide elementary students practice in making maps, while also providing instruction on basic map elements (e.g., scale, symbol, projection), so that students can learn that all maps have inherent inaccuracies and distortions. They also suggested that maps with multiple layers present additional challenges to students.  Therefore, students should engage with maps that have multiple layers and practice making connections between the layers.  However, students should begin with relatively simple layers containing less information, before progressing to maps containing more information-rich layers.[26]
  
  Happily, maps receive the most attention in state social studies standards across the United States, including in Michigan. In forty-two states students are expected to work with maps across elementary, middle, and high school, with no more than two years of unmandated instruction. In Michigan, for example, students are expected to work with maps in every grade from kindergarten through 12th grade, and because Michigan uses process and skills standards that are supposed to be implemented across grade levels and disciplines, it could be argued that they are expected to work with maps in every social studies class as well. Across the United States, however, most map instruction is recommended to take place within the context of geography, and only 17 states address maps across all core social studies disciplines. Notably, 11 states make no mention of maps within the context of history, 30 make no mention of maps within the context of civics, and 32 make no mention of maps within the context of economics.[27] 
  
  State standards documents tend to pay attention to graphical conventions in maps far more than they do in timeline or graphs and charts. In Michigan, for example, 2nd graders are expected to “construct maps of the local community that contain symbols, labels, and legends denoting human and physical characteristics of place.” They are also expected to “use maps to describe the spatial organization of the local community by applying concepts including relative location, and using distance, direction, and scale.” This level of detail in describing visual elements and recognizing conventions is not provided in any standards that include references to timeline or graphs and charts.[28]  
  
  There is less attention paid throughout state standards documents to introducing students to increasingly complex maps; only five states indicate this through references to specific types of maps. Michigan fairs rather well on this point—they specify that students work with “simple” maps in early elementary school, learn about “thematic” maps starting in 3rd grade, and begin working with “data maps” in 6th grade. Notably, however, students may start encountering data maps earlier than 6th grade, so in this way the specific recommendations may actually prove a disadvantage if teachers think they don’t have to teach such maps and leave students to their own devices.[29]
  
  Sadly, only eight states recommend that students should know how to critically analyze maps by the end of high school. There is little attention across state standards to the fact that maps are created by individuals that make choices about what to include and exclude, contain distortions, and can sometimes present data that should be questioned and interrogated. Nor is there sufficient attention in standards across the United States to the fact that maps are projections that can distort data. Again, Michigan does relatively well here, in that they require high school students “evaluate data presented in social science maps considering the origin, authority, structure, and context of the information.” However, Michigan does not make specific recommendations that teachers provide instruction on map projections and distortions.[30] 
  
  As with timelines, and graphs and charts, research on benefits and challenges related to maps in social studies indicates that they are important to teach, and that teachers can’t completely depend on state standards documents to provide instructional guidance. The next module provides some suggestions for helping students analyze and make meaning of different kinds of data visualizations.  
   
• 1 2021-06-21T15:33:53-04:00 How do students learn with data visualizations? 1 Reading data visualizations in print and online social studies texts can improve students' overall comprehension and quality of reasoning. And there is evidence to suggest that reading data visualizations helps students better understand historical and geographic context, multiple causation, and change over time — all important concepts for students to grasp in social studies subject areas.  However, students may face significant challenges in trying to make sense of different kinds of data visualizations. This section provides insight into both benefits and challenges of reading timelines, maps, and graphs and charts. plain 2021-06-21T15:33:53-04:00 In Module 2: Why teach data literacy in social studies?, you read that students can benefit from paying attention to data visualizations that accompany verbal text. In social studies textbooks alone, 90% of data visualizations provide students with extensional information not found in the main body of verbal text.[1] These data visualizations might extend the verbal text by providing geographic contextual information, illustrating changes or movement across space and time, or providing evidence for an argument or explanation. [2] As several scholars have argued, reading data visualizations and other visuals can improve overall comprehension and quality of reasoning, so failure to attend to these data visualizations may hamper students’ understanding of the information provided in the text.[3] Moreover, data visualizations might contain critical background knowledge that students can later use to understand references to people, places, events, or documents.
  
  Unfortunately, research indicates that most students don't pay attention to data visualizations. For example, in a think aloud study that had elementary, middle, and high school students reasoning about a historical question with a history textbook, 74% of the students ignored the data visualization in the passage they were reading, despite the fact that the data visualization was directly related to the question they were trying to answer. Yet, when asked to read the data visualization, most of the participants said it was helpful for answering the question.[4] 
  
  Even if students do pay attention to data visualizations in texts, they may face significant challenges when trying to make sense of them. For teachers, it's important to be aware of these challenges, so that students may overcome them and reap more of the benefits.
  
  Each section below provides a summary of some of the benefits and challenges associated with timelines, map, graphs and charts. 
  
• 1 2021-06-21T15:33:53-04:00 Timelines: Benefits and Challenges 1 Timelines are temporal data visualizations that graphically display historical events in chronological order. Students across the United States are required to work with timelines as early as kindergarten. Working with timelines can help students understand important historical concepts, but they can also present challenges as students try to make sense of them. plain 2021-06-21T15:33:53-04:00 Timelines are temporal data visualizations that graphically display historical events in chronological order. State standards documents from 25 states across the United States required that teachers introduce students to timelines as early as kindergarten or first grade. In Michigan, students are introduced to timelines in kindergarten, where they are expected to create timelines using events from their own lives. And in second grade they are expected to distinguish between "years" and "decades" in order to demonstrate "chronological thinking." As you may have learned while completing the standards analysis exercises in Module Two, Michigan's direct treatment of timelines is sporadic (i.e., they are only mentioned in kindergarten, 2nd, 3rd, and 7th grades), but the standards nonetheless require that students learn about them, and in particular, that they learn how to create them.[5] The question is why should students create timelines? How can the exercise of creating timelines help students learn? 
  
  Creating Timelines to Teach the Concept of Historical Significance
  As soon as a person begins making a timeline, they begin making choices about what to include — and choices about what to leave out. In other words, they make choices about what events or developments are historically significant. And the concept of historical significance is a really important aspect of historical thinking. It is at the root of understanding what the discipline of history is and how it is constructed. As Stephane Levesque argues:

  Because they cannot study everything that happened in the past, historians are necessarily selective in their own investigations. Certain historical events, personages, dates, or phenomena are more important to their studies than others...History, to be meaningful, depends on selection and this, in turn, depends on establishing criteria of significance to select the more relevant and to dismiss the less relevant.[6]

  Just think about this: Have you ever felt upset that you didn't learn about something in a history class? Have you ever heard someone else complain about what wasn't covered in their history classes? Or have students ever asked you why you didn't learn about a historical topic? The topic probably wasn't addressed because the historian or teacher had a different idea than their audience about what is historically significant. Or it wasn't addressed because the topic wasn't deemed historically significant at the time — after all, our ideas about historical significance change with time and place. According to Levesque, historians make choices about historical significance based on the following criteria:

  • Importance - Contextualizing the past and considering what was perceived as important to those who lived at the time, irrespective of whether their judgments about the importance of the event were subsequently shown to be justified.
  • Profundity-How deeply people were affected by an event.
  • Quantity-The number of people affected by an event.
  • Durability-How long an event endures. 
  • Relevance-Relation to current interests, accepting the fact that we cannot divorce ourselves from being social actors in the present.[7]

  This is not necessarily how non-historians make choices about historical significance. As Levesque also argues, non-historians may make choices based on the following:

  • Intimate interests-Is this important to me personally?  Do my loved ones have an intimate attachment?  
  • Symbolic significance-Selection of some events for patriotic or collective justification.
  • Contemporary lessons-Use it to apply to the present. [8] 

  And according to researchers Lauren McArthur Harris and Brian Girard, teachers make their own choices about instructional significance.[9] These include historical considerations, but also contextual and student considerations:Creating timelines offers students an opportunity to grapple with the concept of historical significance. In this activity, for example, students consider their own criteria for historical significance, compare it with historians' criteria, and then create a poster for "determining historical significance."  Using an activity like this early in students' historical studies will prepare them for creating timelines for state, U.S. and world history, and it will help them analyze others' timelines knowing that they made choices about historical significance too.
  
  Analyzing Timelines for Continuity and Change or Cause and Consequence
  So what about analyzing timelines? Is this an activity that can benefit students' learning? Interestingly, state standards documents do a pretty poor job of addressing analysis of timelines. In fact, only seven states explicitly require students to analyze timelines. In Michigan, for example, analysis of timelines is only mentioned in 2nd grade, where students are supposed to use timelines to distinguish between years and decades. Seventh grade is the last time timelines are mentioned at all.  But timelines are everywhere in social studies texts. In a study of commonly used social studies textbooks, Shreiner found that timelines made up 14% of the nearly 3000 data visualizations contained in the textbooks. Even if state standards do not require students to analyze them, students are likely confronted with them on a regular basis.[10] 
  
  And there is much to be gained by providing students with guidance in reading and analyzing timelines. Analyzing timelines supports chronological reasoning about the past. In particular, it can help students identify continuity and change, as well as explore cause and consequence — two other historical concepts that are really critical for historical thinking. Continuity and change is about seeing history as more than a list of events, but as a series of related developments that either signal stability over time, or that show transformations or shifts over time. Recognition of such relationships is what has allowed historians to colligate, or group together, separate events into outwardly simple terms that actually denote complex or long-lasting processes — terms like "industrialization," "urbanization," or "revolution." Teachers can give students tools to analyze such relationships among events on timelines, helping them to recognize how and why events are related, what large-scale processes they might represent, or to what degree a sequence of events represents continuity or change. 
  
  Analyzing timelines can also provide students with insight into historical causes and consequences. Studies of students' thinking indicate that their explanations for why things happened are likely to be mono-causal — that is they tend to think one event led to another, which led to another, and so on, in what we might call a mono-causal chain. In some ways, uncritically viewing timelines — which are often represented as a simple line of events — can exacerbate such misconceptions. However, historians know that multiple causes and multiple consequences are almost always appropriate in explanations of the past. And causes can be grouped in any number of ways: political, social, or economic; proximal or distal; short-term or long-term; as playing the role of trigger or catalyst; or as more or less important. Timelines, such as the ones students find in their textbooks or interactive ones like the WWII timeline to the left, can serve as visual aids to help students both see a sequence of related events, and determine how the events are related in terms of cause and consequence. Students can annotate or categorize related events and then use their analysis to build arguments about an event or phenomenon of the past. Or they can build their own complex timelines that visually capture some of the nuances of cause and consequence in history, showing their causal categories using different symbols, colors, or timeline layers. 
  
  Unpacking Chronological Conventions and Temporal Concepts
  But let's think further about the timelines students see in print and online texts.  Are they easy to read? Even though they may seem pretty simple and straightforward, there are aspects that can be quite confusing, or even misleading. Consider the following timeline, which is from a popular world history textbook. Is there anything confusing about this timeline?Aside from the obvious editorial errors (10,000 what?), this timeline provides a glimpse of the many different chronological conventions — ways of indicating moments in time or the passage of time — that students have to interpret.  Readers see "years ago" on one end of the timeline, and the years themselves on the other end. They also see the notation "ca.," the use of the word "since," and period of time indicated by a year-year notation. That is a lot to unpack and interpret.
  
  When trying to extract information from a timeline, a reader must be aware of all the chronological conventions that denote quantities and passage of time. These include terms like decades or centuries, as well as notations such as BCE (Before Common Era) and CE (Common Era), or BC (Before Christ) and AD (anno Domini — Latin for "In the Year of the Lord"). The notation ca. means circa, indicating historians aren't exactly sure of when an event occurred. When you're dealing with timelines on a very large scale, they may use the notation BP for "before present" or YA for "years ago." These chronological conventions are not really intuitive for students and can therefore be confusing, hindering their ability to reason chronologically.[11] 
  
  The timeline above also demonstrates an effort to represent duration, or how long a historical event or phenomenon lasted, by writing year X-year Y as one piece of temporal data on the line. Not all timelines display duration of events; sometimes they only display points indicating when events begin or end, even though historical phenomena can vary in duration from hours to hundreds or thousands of years. This may be confusing or misleading for students. Even when timelines try to show duration, they can do it poorly.  Consider a timeline that uses bars to represent duration — are they using the same scale for all the bars? Say, 1 cm = 1 year? Are they really helping students conceptualize the difference between a 1-year event, and a 100-year event? And have they clearly represented something that lasted 200,000 years, like the time that Homo sapiens survived as foragers?
  
  Duration is one of several temporal concepts — along with periodization, sequence, and concurrence — that historians regularly use to make sense of and study time, but can be really challenging for students to grasp. For example, historians regularly use a device called periodization to compartmentalize the past into different periods, eras, or epochs based on patterns of change or trends that they have identified as significant. Timelines often display events that characterize a historical "period," but the concept of a historical period alone can be puzzling if one does not understand what defines a period, or how it has been determined. Such confusion might be exacerbated by the fact that historians often disagree over periodization schemes. Concurrence, when events overlap in time, is another important temporal concept for reasoning about the past, and one that is not always well represented on timelines. Events on a timeline may appear to be purely sequential, as if events were one after the other, even when they overlapped in real time.[12] 
  
  Perhaps one of the best ways to help understand these different conventions and concepts is to have them create timelines on their own, allowing them to grapple with some of the challenges inherent in making visually accurate timelines. And by understanding these conventions and concepts, students will be better prepared to analyze existing timelines, questioning what they show and don't show, and using chronological reasoning to examine how events of the past are related.