Interpreting Case and Testing Data by ZIP Code
There are pros and cons to providing disease data by ZIP code. One of the main benefits is that people generally know what ZIP code they live in, while they probably don’t know their census block number. This helps make the data relatable at this very local level. Most of the negatives of this level of data have to do with the added complexity. In order to make it easier to interpret these data, it’s helpful to know that:
- VDH has county data on every case and laboratory report, but we do not have ZIP code data on every record. Because this field is not complete, the total number of cases and tests by ZIP code may not match the same total count by county.
- VDH is reporting testing encounters among out-of-state residents who are tested in Virginia. These tests will not be included in data presented by ZIP codes in Virginia. For more about our testing data, see this blog post.
- ZIP codes are primarily used by the post office for delivering mail. The most efficient mail delivery routes don’t necessarily care about city and county borders. Just like you might have a mailing address for a city that you don’t live in, ZIP codes sometimes cross county and census block lines. This can make it difficult for public health to look at rates per 100,000 population, which is one of the ways we look for patterns in disease data that prompt action.
- Because these data are provided at such a local level, VDH needs to take extra steps to protect the privacy of personal health information. To do this, we suppress case counts of 1-4. We are not suppressing small case counts by city/county because the smallest counties still have enough people in them to provide some privacy.
- Some ZIP codes are mostly industrial or commercial, so there aren’t a lot of people living in them. These ZIP codes may be grouped together with other ZIP codes to provide a better picture of a residential area. VDH is doing this by presenting ZIP Code Tabulation Areas, or ZCTAs. You can read more about ZCTA methodology on the Census website.
- Some ZIP codes may seem surprisingly populous because they serve as the mailing address for a large group of people. College campus mailboxes and P.O. boxes are both examples of this.