Summary
The understanding and interpretation of a concept or a perception may vary from respondent to respondent, hence its measurement may not be consistent. A variable has some basis of classification and hence there is far less inconsistency in its meaning and understanding. Concepts are mental perceptions, whereas variables are measurable either subjectively or objectively on one of the measurement scales. When you convert a concept into a variable you classify it on the basis of measurement into categories, thereby minimizing the inherent variability in understanding. When you are unable to measure a concept directly, you need first to convert it into indicators and then into variables.