Definition: The Constant Sum Scaling is a technique wherein the respondents are asked to allocate a constant sum of units, such as points, dollars, chips or chits among the stimulus objects according to some specified criterion.
In other words, a scaling technique that involves the assignment of a fixed number of units to each attribute of the object, reflecting the importance a respondent attaches to it, is called as constant sum scaling. For example, Suppose a respondent is asked to allocate 100 points to the attributes of a body wash on the basis of the importance he attaches to each attribute. In case he feels any attribute being unimportant can allocate zero points and in case some attribute is twice as important as any other attribute can assign it twice the points. The sum of all the points allocated to each attribute should be equal to 100.
Once the points are allocated, the attributes are scaled by counting the points as assigned by the respondents to each attribute and then dividing it by a number of respondents under analysis. Such type of information cannot be obtained from rank order data unless it is transformed into interval data. The constant sum scaling is considered as an ordinal scale because of its comparative nature and lack of generalization.
One of the advantages of the constant sum scaling technique is that it allows a proper discrimination among the stimulus objects without consuming too much time. But however, it suffers from two serious limitations. First, the respondent might allocate more or fewer units than those specified. Second, there might be a rounding error, in case too few units are allocated. On the other hand, if a large number of units are used then it might be burdensome on the respondents and causes confusion and fatigue.