This guide is intended to increase researchers' awareness of existing resources that can help them define variables to support secondary analysis of administrative datasets. Administrative data are often difficult to analyze because the same or very similar variables are defined differently in different datasets. For example, something as seemingly simple as the age of a child could be defined using a child's exact date of birth, pre-defined age categories (i.e., infant, toddler, preschool-age, etc.), or number of years old at enrollment date (e.g., 3 years old, 4 years old, etc.). These variables may be defined differently among existing administrative datasets. In these instances, researchers must determine the best way to define variables (which may include recoding existing data) before using them. Although there are resources and tools available to help researchers make decisions to resolve these discrepancies, researchers may not be aware of them. This guide summarizes the resources specific to early childhood data and gives examples of how researchers may want to use them. (author abstract)