Final answer:
An observational study is a type of nonexperimental scientific investigation that measures characteristics in a sample without manipulating variables. It is used when experiments are not feasible or ethical.
Step-by-step explanation:
An observational study is a type of nonexperimental scientific investigation where the researcher measures characteristics in a sample but does not manipulate or control variables of interest. It is used when experiments are not feasible or ethical, such as studying the effects of malnutrition on elementary school performance or the relationship between smoking and lung cancer. In contrast, an experiment involves applying a treatment to a group of subjects to isolate the effects of the treatment on an outcome variable.