Most users are not just interested in what the statistics look like for a topic right now; they want to compare them with data from the past. Well, there are many variables one must consider when comparing data over time. Some of these differences include:
Changes in the survey/sample size or universe
Changes in the forms or mediums used to collect data
New decisions on how to tabulate data
New questions or a new order to old questions
Various geographies change over time (towns expand and contract, etc.)
For example, when comparing data from one decennial census to the next, such as Census 2010 to the 2000 Census there are several differences that stand out right away:
Census 2010 allowed responders to mark more than one race for the first time resulting in over 126 different racial/ethnic group combinations
Census blocks (lowest level of geography available) were completely renumbered and switched from three digits to four digits
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