What's the difference between an ecological study and a cross-sectional study? And could you provide an example?
An ecological study is one where you take values of a variable for an entire population, the outcome from that population, and use that to draw inference. A cross-sectional study is where you look at individuals within a population at a single point in time.
Examples: Ecological Study: Consider a study where you find the average number of years of education each of the 50 states in the U.S., as well as their rate of teenage pregnancy, and drew a correlation between them.
Cross-Sectional Study: You recruit people in Montana, and ask them about a number of biological, sexual and behavioral variables and their HPV status, and examine whether particular variables are associated with having (or not having) HPV.