Final answer:
Descriptive research aims to describe and explain phenomena, using methods such as observation, surveys, and case studies. Reactivity, or the presence of an observer, can distort natural behavior, posing a problem for external validity.
Step-by-step explanation:
The basic purposes of descriptive research are to describe and explain phenomena or events and to observe and measure variables. The methods of descriptive research include observation, surveys, and case studies. Observation involves watching and noting behavior or phenomena in their natural setting. Surveys gather data through questionnaires or interviews, while case studies involve in-depth analysis of a single subject or a small group.
Reactivity refers to how the presence of an observer can affect the behavior of those being observed. In descriptive research, reactivity can present a problem for external validity because it may distort or alter the natural behavior of the participants, making it difficult to generalize the findings to a larger population. For example, if people know they are being observed, they may behave differently than they would in their normal environment, resulting in biased data.