Advantages:1.Many social events are such that they have already occurred and cannot be repeated or replicated. Causes behind many events cannot be created or reproduced. Hence conducting experiment on them is not possible.
2. Ex post facto research method has proved it usefulness in the study of changes in average weight height athletic performance morbidity mortality fertility etc. By comparing the present populations with preassembly comparable population that lived in the previous times. It can also the successfully to study changes in crime rates in marriage rates in divorce rates in schooling traffic accidents alcoholism drug addiction etc.
3. With precaution studies through ex post facto design can be approximated to those of experiment.
4. In those cases where before comparability can be ascertained this method is as valid as and as useful as the before after experimental research design.
In some social experiments even it creation of variable becomes possible it takes a long time for the effect to become evident. For example we might want to know the effect of better housing conditions on lowering the amount of delinquency. It may take years before the difference is significantly evident.