Calculating a percent of a margin of error

elizfelter over 6 years ago

Dear community,

I am looking for guidance on calculating a percent of a margin or error. My work involves analyzing potential population risk to flooding, where I geospatially overlay block groups with flood zones in ArcGIS. If, for example, a block group is 75% covered by a flood zone, can I apply that percent to both the population estimate and the margin of error?

Thanks very much for your help!

Elizabeth

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Andrew Beveridge over 6 years ago

There is a formula for combining margins of error. One combines the groups and should generally recompute the standard error, which will be much smaller for larger areas. If the area is smaller than the entire area, the standard error should be higher. In the simplest terms (the Census uses complex sampling for the ACS) the standard error is function of the square root of the total. So if your sample is based upon 100 and the margin of error is 16%, if your sample is based upon 400 the standard error becomes 8%, etc. Complex sampling increases the standard error, by effectively shrinking the sample size by the design effect.
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Andrew Beveridge over 6 years ago

There is a formula for combining margins of error. One combines the groups and should generally recompute the standard error, which will be much smaller for larger areas. If the area is smaller than the entire area, the standard error should be higher. In the simplest terms (the Census uses complex sampling for the ACS) the standard error is function of the square root of the total. So if your sample is based upon 100 and the margin of error is 16%, if your sample is based upon 400 the standard error becomes 8%, etc. Complex sampling increases the standard error, by effectively shrinking the sample size by the design effect.
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