Dr. Rhodes is interested in how differing levels of light affect how people perceive color. He finds participants for this research study by making an announcement in several psychology classes at his university. Should Dr. Rhodes be concerned about using this sampling method for his research study?

convenience sampling

Explanation:

The convenience sampling is described or explained as the method of sampling  that involves the taking of data from the group of individuals that is close or that are conveniently accessible to the researcher.

In this case, Dr Rhodes made use of the subjects or participants that are very close to him for his research, this he did by announcing at several psychology classes at the university where he is working.

could you go a bit more in depth with you question? it's quite hard to understand. as soon as you do that i will be glad to : )