A longitudinal research is an observational research method in which information or data is collected for the same subjects over a period of time. It is a type of correlational research that includes looking at variables or samples over an extended period of time. In correlational research is a non-experimental study based on describing relationships among variables and making predictions. They are often used in the field of psychology. Longitudinal researches can take place over a week, month, year or even years. In some cases, it can last for decades. The data of longitudinal study can either be quantitative or qualitative. In this research, the survey respondents are observed over a period of time ranging from weeks to even years to observe any noticeable changes in them or in their behavior. In a longitudinal research, same variables are being observed over the study period. These researches are effective to study the changes that happen over a period of time. A longitudinal research can be applied in various fields. There will be myriad of reasons why a researcher may want to conduct longitudinal study. The Terman Study of the Gifted, also known as the Genetic Studies of the Genius, is the oldest and longest running longitudinal studiy in the world. The study, which began in 1921, was aimed to observe the development and characteristics of gifted children over the lifespan. It was conducted by Lewis Terman.
There are mainly three types of longitudinal study. They are Panel study, Cohort study, and Retrospective study.
- Panel study– They are a particular type of study in which there will be samples of individuals from a huge population. This study is conducted at specified intervals for a longer period of time. The collection of data is often called as waves. One of the important characteristics of panel study is that data is obtained from the same individual at different points in time. Some of the major panel studies include Baltimore Longitudinal Study of Aging, The Berlin Longitudinal Study, Panel Study of Income Dynamics etc.
- Cohort study– Cohort is another type of longitudinal study that samples a cohort (a group of people who have experienced a common event in a given point of time). Specifically cohort studies recruit and follow individuals who share a common characteristic such as demographic similarity or a particular occupation. In this study, selecting a group of people will be based on a specific event such as birth, historical experience or geographic location. Usually, these studies adopt a purely observational approach.
- Retrospective study– Retrospective is a form of longitudinal study that makes use of already existing data, which exists because a similar type of research had been conducted recently. This study involves looking to the past by checking historical information such as medical reports. The researcher use administrative database that already exists while conducting a retrospective study.