In analyzing statistical results, it is important to make sure that "apples are being compared to apples"—meaning that an apple is defined clearly in both data collection and data reporting to mean the same thing. When possible, variables should be defined in a way that is consistent with commonly used definitions or classification systems. Some of these systems apply cross-nationally, particularly in the area of international trade, while others are set at the national level. Two examples from North America:
Use of such standards helps ensure comparability of results across surveys and time. When evaluating a dataset, be sure to consider whether the variables have been standardized, so that you can be confident in the comparability of the statistics you use in your analysis!