Why can’t 2020 or 2021 data be pooled with data from other years? What about 2015 data?

2020 estimates cannot be pooled or combined with other years of data. This is because of major changes in survey mode (i.e., web versus in-person data collection) caused by the Coronavirus 2019 pandemic. In 2019, data collection was exclusively in person, but in 2020, most respondents in Quarter 4 answered via the newly introduced web survey. People answer differently in person and via the web, both because different people answer the survey and because they think about the questions differently in different contexts. This is called a mode effect.

The mode effect is not the same across different groups. As a result, comparing averages that include 2019 and 2020 data does not produce valid results. The differences are inconsistent across measures, so they cannot be accounted for with weighting.

Because of changes to the prescription drug use questions in 2015 and context effects, we also caution against pooling 2015 data with prior years, and pooled estimates were not published.