Question

In order to measure aggressiveness in toddlers, a researcher sat on a park bench for an afternoon and observed children playing on the slides and climbing equipment. To get clean results, it was very important that the toddlers not know that someone was observing them. What type of experiment would best fit these criteria?

Answer 1

The answer is a Natural Experiment.

A natural experiment takes place in natural settings and are used when independent variables can't be manipulated directly for ethical/practical reasons, so in this case the toddlers playing.  Any effect observed happens naturally. They aren't seen as true experiments as the independent variable (toddlers playing) hasn't been changed deliberately to see effect on dependent variable (aggressiveness).