How can you compare the development of an unusual group with that of the norm group?
In statistics, the more complex the model, the larger the sample needs to be in order to obtain effective estimates and reach statistically significant results. One example of a complex model is a latent growth model (LGM): a model for estimating growth that can also be used to compare groups. For research questions that fit this model it is not always possible to find sufficient numbers of participants. Sometimes the population is limited or there are ethical considerations that restrict the sample. For example: if you wish to compare the development of people with a rare illness with the development of people who do not have the illness. Or: if you want to compare the development of under-age girls who have committed murder with girls who have committed other crimes.
Research has shown that a special branch of statistics, Bayesian statistics, sometimes offers greater options than conventional statistics. In Bayesian statistics, researchers involve prior information in the research.