Qualitative variables are those with no natural or logical order. While scientists often assign a number to each, these numbers are not meaningful in any way.
Examples of qualitative variables include things such as color, shape or pattern. For instance, if scientists were studying a group of people, they may want to record the skin color of the subjects. However, the various skin colors are not numerically related to each other. Similarly, the scientists may wish to record the race of the people; however, the various races are not related to each other numerically. By contrast, quantitative variables do have a natural order. For example, height, weight or density are all quantitative measurements. While quantitative variables offer the ability to obtain more detailed analysis, they are not intrinsically superior to qualitative variables.
Qualitative variables are more common in some disciplines than others. Particle physicists, for instance, do not often work with qualitative variables. On the other hand, researchers in the fields of psychology, biology and sociology are often forced to use qualitative variables in their work. Sometimes, this has been used as justification for calling such disciplines “soft” or “descriptive” sciences, rather than “predictive” sciences, such as chemistry and physics.