A sequential study is one of many ways to construct research studies. Sequential, or longitudinal, studies test a single variable on the same individual or group of individuals consistently over a period of time. Other ways of constructing a research study include surveys, experiments and cross-sectional studies. Each of these methods has advantages and disadvantages.
Strength: Mitigated Cultural Variations
By consistently studying the same group of people, researchers are able to eliminate cultural or demographic factors from their findings. While variations may exist within the study group, these variations will persist from one measurement to the next. This is in contrast to a cross-sectional study done multiple times, where the researcher measures the same variable but takes a different sample each time.
Weakness: Participant “Mortality”
One weakness that plagues longitudinal studies is the steady decrease in participation over time, referred to as “participant mortality.” The number of subjects able to participate decreases with each survey, particularly when studies occur over years or decades. As a result, many critics contend that the survey results toward the end of a sequential study may be measurably different than the overall group that began the study.
Strength: Observing Changes
Controlling for the cultural differences and time allow sequential studies to more accurately measure changes than other types of studies. For example, asking the same group to rate, on a scale from one to 10, how much they trust the local news anchor can demonstrate how this trust changes from youth to old age. When you apply the previous example to a diverse range of ages, this may accurately show how different times of the year or environmental factors, such as economic changes, might have a broad-reaching impact on the response. If researchers construct the study properly, environmental factors should affect the group evenly, mitigating individual or cultural variations.
Weakness: Poor Causational Analysis
To find the cause behind a phenomenon, studies will manipulate a factor and record the results; however, most sequential studies merely observe the subjects without manipulating environmental factors. Therefore, the cause behind a trend -- for instance, the previous example demonstrating a growing distrust of media -- can be difficult to ascertain.