A large representative random sample of 6906 U. S. adults collected over 20 years showed that "parents reported higher levels of life satisfaction than nonparents," with the observed difference in life satisfaction between the two groups being statistically significant. This is an example:

Observational study

Explanation: Observational study involves making conclusion or generalization about a whole or larger group from a smaller group which is representative of the larger group. In some scenarios such as in the question above, sampling the whole population of adults in the United States might be practically impossible due to several constraints. Therefore, a large representative random sample is employed in the study and the conclusion of the research study is used to draw inference about the entire adult population of the United States.