Definition (1):
A research method that combines cross-sectional and longitudinal research by considering a number of different age groups and examining them at several points in time is called sequential research.
For example, investigators might use a group of 3, 5, and 7-year-olds, examining them every six months for a period of several years. This technique allows a developmental psychologist to tease out the specific effects of age changes from other possibly influential factors.
Definition (2):
“Sequential research is that which is carried out in a deliberate, staged approach [i.e. serially] where one stage will be completed, followed by another, then another, and so on, with the aim that each stage will build upon the previous one until enough data is gathered over an interval of time to test your hypothesis.”
Under sequential research, the size of the sample is not pre-determined. After analyzing each sample, the researcher is able to accept the null hypothesis, or reject it, or accept the alternative hypothesis, or choose another set of subjects and perform the study once again. It means s/he can get an unlimited number of subjects before s/he can make a final decision on which hypothesis to accept.