Qualitative analysis is the scientific study of data that can be observed, but not measured. It is concerned with cataloguing the qualities of what is studied. Quantitative analysis is the study of data that can be measured, the quantities of a category of data. Each type of inquiry contributes important insights to scientific study that the other cannot.
A scientific study usually employs both methods of observation, but will tend to rely more heavily on one or the other dependent upon the subject of the study. In the field of psychology, there are many quantities that can be measured. However, there are many nuances that would be missed if the researcher were to rely too heavily on quantitative data alone.
The quantitative data provides the framework of the study. Many times a study is only concerned with a particular gender and age group, such as males from 18-35 years old. A certain condition or its absence might be another determining factor. All of these factors can be measured. For example, out of 200 males chosen for the study, 40 percent were older than 25 and 75 percent had the condition being studied. Over the course of the study, more quantitative data would be gathered, such as how many subjects improved or showed a certain side effect. There would also be qualitative data collected. How well the subject was feeling or which side effect he was experiencing may not be able to be objectively measured. Each subject would likely describe his experience differently, and the researcher would look for patterns among the descriptions.