https://www.bmj.com/about-bmj/resources-readers/publications/epidemiology-uninitiated/7-longitudinal-studies

The above reading may be relevant. Good luck.

Dear Mst. Sadia Sultana

I hope that what follows can be of help to you.

Your question has to do with methods often used in developmental psychology. As far as I know, longitudinal, cross-sectional and time-lag studies are the main methods to analyzes developmental change. Their differ in several respects.

Longitudinal studies (LS) cross-sectional (CS), and time-lag (TL) studies are different methods in terms of goals to be attained and and procedures to be used.

LS are interested in getting access to intrapersonal change over time; CS are interested in getting access to interindividual differences at a given point of individuals' development. TL are interested in associating a certain age with a certain developing capacity.

Even though both longitudinal and cross-sectional studies are often used in developmental psychology, only the former is consistent with the very nature of developmental psychology, to get access to intraindividual change over time.

Let us suppose that you have a sample of, say, 100 individual who were born in the same year, 2000, for example, and that you are interested in seeing if their cognitive development changes over time. In order to attain this goal, you have to evaluate your sample' cognitive abilities, for example, at T1 (2018), T2 (2020), T3 (2022), and so forth, when they are eighteen, twenty, and twenty-two-year olds. So, in a longitudinal design, individuals of a certain age, 10-year-olds, for instance, at T1, are followed and evaluated when their age is 12-year-olds (T2), 14-year-olds (T3), and so forth. Thus, you have repeated evaluations ( at T1, T2, T3, and so forth) of the same individuals of a given cohort (e.g., those individuals who were born, for example, in 2000) over time. Needless to say, the more a longitudinal study is long, the more likely you are to lose some individuals of the initial experimental sample.

In contradistinction, in a cross-sectional design, you have individuals of different ages (e.g.,10, 12, 14- year olds) who are evaluated at the same time (at T1, for example). In a nutshell, in longitudinal studies, individuals who belong to a certain cohort or have the same age are assessed at several points in their development (at T1, T2, T3, and so forth). In cross-sectional studies, individuals of different ages (e.g., 10, 12, 14- years olds) are assessed only once (at T1, for example) .

In a time-lag design, you can see, for example, the typical cognitive performance in a Piagetian task when individuals are 18 year-olds, for instance. In a time-lag design, however, these individuals have to belong to different cohorts and be assessed at different times.

In multi-wave studies, you still have three types of developmental studies or designs, longitudinal sequential studies, cross-sectional sequential designs and time-lag sequential designs . Each of these types allows you to make longitudinal, cross-sectional, and time-lag evaluations. In the first, you follow different cohorts over a certain period of time; in the second, you assess a cross-sectional sample at different times (T1, T2, T3, and so forth); In the third, you assess a time-lag sample at different times. For a more detailed description of these methods you can read, for example, the following book; Achenbach, T (1978). Research in developmental psychology: Concepts, strategies and methods. New York: The Free Press.

I think the two terms are very similar. I would concentrate instead on your purposes for conducting two studies. For example, are you basically looking at change over time, or was there some kind of intervention so that these are pre-test and post-test data.

Hi Mst. Sultana,

In a longitudinal study, researchers repeatedly examine the same individuals to detect any changes that might occur over a period of time. (Please keep in mind: "repeatedly examine the same individuals"). Now question is - "over a period of time".

so long as the participants are repeatedly observed. They can range from as short as a few weeks to as long as several decades. However, they usually last at least a year, oftentimes several.

On the other hand for a follow-up study: Please see the link:

https://link.springer.com/referenceworkentry/10.1007/978-1-4419-1005-9_314

Now you can differentiate: in the follow-up study no phase "repeatedly examines the same individuals".

For follow-up study you can consider, Base-line and End-line study, Pre and Post study etc.