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Q:  

I am using data from the American Community Survey (ACS) for Monroe and Lenawee counties in Michigan. The tables from American Factfinder have margins of error for all the cells. However, if I am combining the two counties is there a way to calculate new margins of error based on this larger population?

A:  

The best description of the way to do this is in this technical appendix:

Understanding and Using ACS Single-Year and Multiyear Estimates

Appendix 3 has several examples of computing margins of error when one is combining geographies (counties, census tracts, etc.).

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