Drilling down to ZCTAs in NHGIS



I am new to NHGIS but not ACS. I would like to use NHGIS to produce the number of Medicaid enrollees in each ZCTA by year. I was able get enrollment estimates for the 2011-2015 ACS.

A few questions…

  1. Since I used the 5-year ACS to get these estimates, are the zip-level enrollment counts accurate down to the census tract level for the 5-year period?

  2. Is it possible to get accurate 1-year estimates of zip-level enrollment counts usnig NHGIS?

I’m guessing the answer to (2) is no, but I thought I would ask.

Thank you!


  1. You can obtain 5-year ACS summary data for either ZCTAs or census tracts. These levels do not relate to each other in any consistent way. I.e., any given census tract is likely to straddle some ZCTA boundaries and vice versa. The accuracy of ACS summary data for small areas like ZCTAs or census tracts is generally not great, but it can be adequate for many analyses, especially in cases where the counts are relatively large. (Smaller counts tend to be less reliable.) The accuracy of ACS summary data is indicated by the margins of error that are included with any ACS data table from the NHGIS or the Census. See this Census web page for more information on using ACS margins of error.

  2. No. The Census provides 1-year ACS summary data only for areas with at least 65,000 residents. Because ZCTAs generally don’t have that many residents, the Census supplies no 1-year data tables for ZCTAs. NHGIS redistributes only tables that the Census supplies, so NHGIS has no 1-year ZCTA data either.