A case study is a rigorous academic work or research about a single person or group, in order to gain knowledge about certain thoughts or particular behaviors.
Researches are often careful about surmising the results of studies, because sometimes is almost impossible interviewing every member of a studied population; thus researches must select a sample of it.
A random sample is a set of participants haply determined, so the population is equitably represented; a wrongly picked sample might except key elements of the population.
When participants of a study are volunteers that have different attributes than people who do not volunteer, then "Volunteer Bias" take place; and this can slant the results of a study.
"Earlier studies were more slanted toward Caucasian males" due segregation and other social conditions taking place heretofore; so opportunities were less equitable.