People always think they are right. Agree?  

While expressing opinion people over stress their point of view. People talk about the positive aspects of their decisions to show / prove how intelligent they are and also to avoid " being looked as a Fool ". The negative aspects of the decisions will not be discussed. So opinion gathered by talking to people will be biased. Say for example if we talk to someone regarding his choice of place to buy his house, his choice to buy a car, the type of furniture he bought, the types of Building material he used etc … every person will just prove and justify his decision is the best ! This becomes one of the major drawbacks in opinion survey exercises.