A researcher wants to study math achievement in sixth graders. The researcher obtains a list of all elementary school districts in the state, randomly selects 5 percent of the districts, and administers achievement tests to all sixth graders in these districts. This sampling method is called sampling.

This sampling method is called cluster sampling.

Explanation:

In cluster sampling, the researcher will first divide the population into separate groups. The researcher in the passage focused on sixth graders of a certain state as his subjects. The next step is to select a simple random sample of clusters from that population. In this case, the researcher randomly selected 5% of the districts in the state, and administered the achievement test to their sixth graders. Therefore, we can safely say he used cluster sampling.