Hypothesis generating studies include: case studies;

case series; and

cross-sectional studies.

Case studies are in-depth descriptions of the factors related to an illness or injury of an individual in which new possible relationships or causes are found, by history, examination, or testing. Case series are a group of individuals with the same, or similar, illness or injury and with the same, or similar, factors that may be importantly related to the disease or injury. A classic case series in the past was the five chest x-rays of young men in a San Francisco hospital that showed an unusual type of pneumonia, which resulted in the first description of Pneumocystis Carinii Pneumonia and AIDS. What these five young men also had in common was their having sex with other men.