The practice of classic market research teaches you that by-in-large people over inflate their own opinion even to the degree of ignoring data that would tend to contradict that opinion.

Pressure applied by clients to support the client’s desired outcome can result in analyses and conclusions that serve no positive value to the client and often do harm by confirming risky decisions.

The analyst must work to eliminate his/her own bias as well as the bias of the client, though the latter is difficult, and should in all cases refrain from supporting the optimistic outcome.