The longitudinal study designs study the impact of an intervention, phenomenon, situation or treatment over a period of time. The investigator studies the population at one time and makes record of it and then over a period of time he/she continuously collect data from the same respondents to know the impact of time on the impact of an intervention or situation. This is a very useful type of study design for government and private organizations. These organizations collect data over a period of time to know how and when the impact started and how it changed. It is also very useful for studying the health situations of a community.

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Examples
To study the immigration trends of the IT related Indian nationals to USA from 2010 to 2017.
To study the infertility trends in a community in a given time period.
To study the prevalence of breast cancer among northern american females in a certain time period.
To survey the a particular community about first hand evidence of crime in a given time period.
To analyze the domestic violence in California suburbs from 2005 to 2017

Advantages of longitudinal study design
The longitudinal studies are very beneficial for government organizations to collect data about population related to various aspects of life. For example; the health department collects data to know about the changes in the prevalence of a disease after the intervention of a treatment. In this type of study the aim is to know the pattern of change rather than just the intensity of change as in the before-and-after studies. The private organizations also conduct these studies to improve the quality of their product or service. They ask people questions over a long period of time and collects data periodically. These kind of studies have unique advantages that one cannot achieve with other study types.

Disadvantages
The disadvantages of the longitudinal study design is that some time respondents lost interest in the participation as they are long studies. Respondents may answer about the survey questions without thought and carelessly, the respondents should be made aware of the importance of the survey and its significance to make them fully responsible for their answers. Some respondent may not want to participate when the study takes long time and may quit after few sessions which can be very difficult situation for the investigator.