In a well-developed marketing research plan, control over various sources of error should be included.
General error represents the deviation of the true mean value of the variable of interest in the population from the observed mean value obtained through marketing research. For example, according to the latest U.S. Census results, the average annual income of the population is $75,871, whereas marketing research based on a sample survey reports an average annual income of $67,157.
Sampling error occurs because a specific sample is not fully representative of the population of interest to the marketer. Sampling error is the deviation of the true mean value for the population from the true mean value for the initial sample. For instance, the Census data shows the average annual income as $75,871, while results from a sample survey based on a postal panel (considered quite accurate) indicate an average annual income of $71,382.
Systematic errors are unrelated to sample formation; they can be random or non-random and arise from various causes, including errors in problem definition, approach development, scaling, questionnaire structure, interviewing methods, data preparation, and analysis. Systematic errors consist of non-observation errors and observation errors.
Non-observation error occurs when responses cannot be obtained from some respondents in the sample. The main reasons for non-observation errors are refusals and the respondent being unavailable. Non-observation can lead to the actual sample differing from the initial sample in size or composition. Non-observation error is defined as the deviation of the true mean value of the variable in the original, planned sample from the true mean value in the final, actual sample. For example, the average annual income for the initial sample is $71,382, but only $69,467 for the actual sample, with both figures obtained from a postal panel survey.
Observation error arises when respondents provide inaccurate answers, their answers are incorrectly recorded, or they are misanalyzed. Observation error is defined as the deviation of the true mean value of the variable in the actual sample from the mean value obtained in the marketing research project. For example, the average annual income for the actual sample is $69,467, but according to the marketing research, it is only $67,157. Observation errors may be made by researchers, interviewers, or respondents. Researcher errors include errors in information replacement, measurement, population definition, sampling model, and data processing.
Information replacement error is defined as the deviation of the information needed to address the marketing research problem from the information found by the researcher. For example, instead of obtaining information about consumer choice of a new brand (needed to address the marketing research problem), the researcher obtains information about consumer preferences because it is difficult to obtain information on consumer choice.
Measurement error is defined as the deviation of the information sought by the marketer from the information obtained through the measurement process. For instance, although the marketer is interested in consumer preferences, they use a scale that measures perception more than preferences.
Population definition error arises from the deviation of the actual population size relevant to the problem from the population defined by the researcher. Properly defining the population can be far from trivial, as shown in the example of wealthy households.