I am Susan Kistler, the American Evaluation Association’s Executive Director and aea365’s regular Saturday contributor. Last week, at the Minnesota Evaluation Studies Institute, we had a discussion about when to use derived variables.
Lessons Learned – What is a derived variable? Derived variables are variables that are computed based on other variables.
We began by looking at the distribution of AEA members residing in the United States and quickly found that California dominated the states in terms of members residing there. But this didn’t really tell the whole story. The top ten states by total number of members residing in them is shown at right. Having over 700 members in California would be judged differently from having over 700 members in Rhode Island given each state’s population.
It was time to add population to the mix.
First, we divided the total members in the state by the state’s population. This gave us us numbers like 0.0000391204283265605 which, while accurate, were difficult to readily read and interpret. Multiplying by one million resulted in a derived variable representing the number of members per million state residents.
The next graph at right is based on the derived variable, members per million population. California, once so dominant, doesn’t even make this new top ten list and states with smaller populations, such as Alaska, Vermont, and Hawaii, are now reflected more in the mix.
The derived variable in this case helps to make more direct state-to-state comparisons, taking into account population.
Want to check out total membership and member density in your state? Click on the graph below to go to the full interactive option showing every state.
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