the concepts of ____ are important, not only in helping us understand how consumers perceive prices, but also in conducting consumer research to measure the size of the assimilation region around a particular referent price.

The missing phrase is: assimilation and contrast.

These concepts come from assimilation–contrast theory in consumer psychology.

So the full sentence should read:

“The concepts of assimilation and contrast are important, not only in helping us understand how consumers perceive prices, but also in conducting consumer research to measure the size of the assimilation region around a particular referent price.”

This wording appears verbatim as the keyed solution to an NPTEL Consumer Psychology assignment question.