The psychologist is using "random sampling".
Random sampling is a part of the sampling method in which each sample has an equivalent likelihood of being chosen. An sample chosen randomly is intended to be an unprejudiced portrayal of the aggregate populace. On the off chance that for a few reasons, the example does not speak to the populace, the variation is known as an sampling error.
Random sampling is one of the least difficult types of gathering information from the aggregate populace. Under Random sampling, every individual from the subset conveys an equivalent chance of being picked as a piece of the testing procedure.