Researchers can use qualitative research methods such as case studies as a source of new ideas about behavior, tentative support for a theory, and an evaluation of therapeutic techniques. In developmental psychology, case studies might be used to evaluate long-term progress of a psychological issue pertaining to human development. Case studies are useful to a certain extent, but we must remember that they are reported by biased observers and rely upon subjective evidence.
Researchers use quantitative research methods such as correlation studies, because they are more advantageous than qualitative research. Because researchers measure their variables, observe numerous participants, and apply statistical analyses to their research, they are better able to generalize their data to people beyond the paritipants in the study. Additionally, correlation studies can be easily repeated. A correlation study might be used to determine whether there is a correlation (relationship) between reading to your children every night and better school performance.
Mixed-methodology research is the most useful and valid form of research, because it uses both quantitative and qualitative methods to measure information. Additionally, mixed-methodology uses a technique called triangulation which evaluates the consistency of the findings by comparing quantitative and qualitative results to better support the researcher's conclusions. This method allows Developmentalists to understand the unique lives of people while making numerical comparisons to support or disprove hypotheses.