In 1980, the Census Bureau used suppression to protect the confidentiality of respondents. As a result, you will observe situations in the data like you illustrated in your post.
For 1980, the following population information was never suppressed (more details about suppression are available on pages 19-29 of this technical document:
- Total Population
- Hispanic Ethnicity
- Total Housing Units
- Year-round Housing Units
- Occupied Units
- Vacant Year-Round Housing Units
Suppression for other population characteristics, such as sex and age, was applied in various circumstances. For STF1, suppression for these characteristics was applied when a geographic unit had fewer than 15 people. In your example, suppression was applied to the age counts but not the total population, race or Spanish Origin tables.
It’s even more complicated than that, however. When the Census Bureau published their digital data files for 1980, data storage was expensive. In order to minimize space, the Bureau published data for “split census tracts” - these are census tracts that are split by city boundaries. The same suppression rules applied to split tract data.
When we started NHGIS, we realized that we needed to create 1980 data for “whole census tracts”. This required us to aggregate data for split tracts to create whole ones. We used the suppression flags on the split tracts when we created the whole tract data. Thus, if one part of a whole tract was suppressed, we then suppressed data for the whole tract. Thus, when you see a suppression flag for a census tract with a large population, that particular data record was probably created by aggregating split tracts AND one split part had suppression applied. Essentially, one part of the split tract had a very low population (under 15) and its characteristics were suppressed.
We actually provide the “split tract” data that we used to build the “whole tract” data that you’ve been looking at. When you’re on the Data Options page of NHGIS, you can click the hyperlink under GEOGRAPHIC LEVELS. The following popup will open:
If you click the SHOW COMPOUND GEOGRAPHIC LEVELS hyperlink, you will see the list of all “split” geographic levels for the dataset (e.g., 1980 STF1). To get the split tract data, you’ll want to click the button as shown on this screen shot:
When you get the data for this level, you will be able to isolate the “split tracts” and compare their statistics. Many of the parts with large populations will not have suppression applied. If the small part is quite small (e.g., 10 people), then you may be able to just use the larger part for your project.
Dave Van Riper
IPUMS NHGIS research scientist