A single sample of the population that has been identified and has to be stretched over a period of time can called as longitudinal study design. This kind of study pattern has its certain specific features that distinguish it from other studies. These features are:
- The study involves a pattern of choosing a panel of representatives or individuals that represent the universe of the study.
- It involves the measurement of the responses of the respondents, not just once but over and over again at fixed intervals. The measurement is made to collect data for the study of the fixed variables under study.
- A mandatory feature of the study is that once the sample has been selected, for the entire period of the study it needs to stay constant. In simple terms it means that the number of panel members cannot change. Under circumstances that a panel member leaves because of some unavoidable circumstances it has to be replaced by another representative from the population of study.
In order to determine a change or consistency on the variable under study, over a span of time the most appropriate design would be the longitudinal study. They are sometimes also called time series designs as they are considered over a long period of time.
As explained earlier, repeated measurements can be derived from the sample that has been kept constant over a significant period of time. Depending upon the nature of the study, the different groups could be also studied for different stages of the study. Both these kinds of researches would fall under the domain of the longitudinal type of study only but the outcome and the analysis for both would be vastly different.