Know Your Data

Now that you’ve found, sourced, and inspected the file, the next step to know your data more deeply than what appears at its surface level. Read the source notes and examine the contents to reflect on what is explicitly stated—or unstated—to better understand its origin, context, and limitations. You cannot program a computer to do this step for you, as it requires critical-thinking skills to see beyond the characters and numbers appearing on your screen.

One place to start is to ask: What do the data labels really mean? and to consider these potential issues:

  • Abbreviated column headers, such as Elevation or Income, often appear in spreadsheets. Sometimes the original software limited the number of characters that could be entered, or the people who created the header names preferred to keep them short. But was Elevation entered in meters or feet? An abbreviated data label does not answer that key question, so you’ll need to check the source notes, or if that’s not available, compare elevation data for a specific point in the dataset to a known source that includes the measurement unit. Similarly, if you’re working with US Census data, does the Income abbreviation refer to per person, per family, or per household? Also, does the value reflect the median (the mid-point in a range of numbers) or the mean (the average, calculated by adding up the sum and dividing by the number of values). Check definitions in the source notes.

  • Exactly how was the data recorded? For example, was Elevation for a specific location measured by a GPS unit on the ground? Or was the location geocoded on a digital map that contains elevation data? In most cases the two methods will yield different results, and whether that matters depends on the degree of precision required in your work. Similarly, when the US Census reports data from its annual American Community Survey (ACS) estimates for Income and other variables, these are drawn from small samples of respondents for lower levels of geography, such as a census tract with roughly 4,000 residents, which can generate very high margins of error. For example, it’s not uncommon to see ACS estimates for a census tract with a mean family income of $50,000—but also with a $25,000 margin of error—which tells you that the actual value is somewhere between $25,000 and $75,000. As a result, some ACS estimates for small geographic units are effectively meaningless. Check how data was recorded, and any reported margins of error, in the source notes.

  • To what extent is the data socially constructed? In other words, what do the data labels reveal or hide about how people defined categories in different social and political contexts, which differ across place and time? For example, we designed an interactive historical map of racial change for Hartford County, Connecticut using over 100 years of US Census data. But Census categories for race and ethnicity changed dramatically during those decades because people in power redefined these contested terms and moved who belonged in which group. For example, through the 1930s, the US Census separated “Native White” and “Foreign-born White” in its official reports, then combined and generally reported these as “White” in later decades. Also, the Census classified “Mexican” as “Other races” in 1930, then moved this group back to “White” in 1940, then began to report “Puerto Rican or Spanish surname” data in 1960, followed by “Hispanic or Latino” in later decades, as an ethnic category that was distinct from race. The Census finally replaced “Negro” with “Black” in 1980, and in 2000 allow people to select more than one racial category, such as both “White” and “Black,” unlike prior decades when these terms were mutually exclusive and people could choose only one. As a result, historical changes in the social construction of race and ethnicity influenced how we designed our map to display “White” or “White alone” over time, with additional census categories relevant to each decade shown in the pop-up window, with our explanation of our decisions in the caption and source notes. There is no single definitive way to visualize socially-constructed data when definitions change across decades. But when you make choices about data, describe your thought process in the notes.

To be clear, you may never truly know your data if it was collected by someone else, particularly a different person in a distant place or time. But don’t let philosophical obstacles stop you from asking good questions about the origins, context, and limitations of your data. Only by clarifying what we know—and what we don’t know—can we create meaningful visualizations that bring people’s stories behind the data to the forefront.


This chapter reviewed two broad questions that everyone should ask during the early stages of their visualization project: Where can I find data? and What do I really know about it? We broke down both questions into more specific parts to develop your knowledge and skills in guiding questions for your search, engaging with debates over public and private data, navigating open data repositories, sourcing data origins, recognizing bad data, and understanding your data at a deeper level. Remember these lessons as you leap into the next few chapters on cleaning data and creating interactive charts and maps.