So you can have a sample study, and we've already talked about this in several videos, but we'll go over it again in this one. You can have an observational study. Observational study. Or you can have an experiment. Experiment. So let's go through each of these, and always pause this video and see if you can think about what these words likely mean, or you might already know. Well, sample study we have looked at, this is really where you're trying to estimate the value of a parameter for a population.