The first job of a market researcher is to eliminate bias.  This is very difficult to do and requires rigor and consistency in methodology.

Examples of bias include:

Random Sampling Error: A statistical fluctuation that occurs because of chance variations in the elements selected for the sample.

Systematic Error: Error resulting from some imperfect aspect of the research design that causes respondent error or from a mistake in the execution of the research.

Sample Bias: A persistent tendency for the results of a sample to deviate in one direction from the true value of the population parameter.

Respondent Error: A classification of sample biases resulting from some respondent action or inaction such as nonresponse or response bias. (Qualitative questions on a survey are prone to this error).

Nonresponse Error: The statistical differences between a survey that includes only those who responded and a perfect survey that would also include those who failed to respond. (It is crucial to include the effect of nonresponse)

Self Selection Bias: A bias that occurs because people who feel strongly about a subject are more likely to respond than people who feel indifferent about it. (This is the primary problem with focus groups).

Response Bias: A bias that occurs when respondents tend to answer questions in a certain direction that consciously or unconsciously misrepresents truth. (This is why a variety and a lot of quantitative questions should be asked).

Other biases include:

Acquiescence bias, Extremity bias, interviewer bias, auspices bias, social desirability bias

Bias is always the enemy in market research.