Comparative Analysis is used when you have to compare two objects from two different sets of Objects or compare one Object with Full set.
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Comparative analysis is used to analyse data based on different group of values of same dimension.
For example :
If you want to Analyse Sales of two set of countries , make group for each set of countries. Then make one group as alternate state. Then create expression for measure for each group. Now select values from each group and see the difference in corresponding values .