Elementary Statstics

3.1.14.1.1 Causes of Bias

Bias may arise due to:

(i) in appropriate process of selection

(ii) in appropriate work during the collection; and

(iii) in appropriate methods of analysis

(i) Faulty selection. Faulty selection of the sample may give rise to bias in a number of ways, such as:

(a) Deliberate selection of a ‘representative’ sample.

(b) Conscious or unconscious bias in the selection of a ‘random’ sample. The randomness of selection may not really exist, even though the investigator claims that he has a random sample if he allows his desire to obtain a certain result to influence his selection.

(c) Substitution. Substitution of an item in place of one chosen in random sample sometimes leads to bias. Thus, if it is decided to interview every 50^th householder in the street, it would be inappropriate to interview the 51^st or any other number in his place as the characteristics possessed by them differ from those who were originally to be included in the sample.

(d) Non-response. If all the items to be included in the sample are not covered there will be bias even though no substitution has been attempted. This fault  occurs particularly in mailed questionnaires method, which are incompletely returned. Moreover, the information supplied by the informants may also be biased.

(e) An appeal to the vanity of the person questioned may give rise to yet another kind of bias. For example, the question ‘Are you a good student?’ is such that most of the students would succumb to vanity and answer ‘Yes;

(ii) Bias due to Faulty Collection of Data. Any consistent error in measurement will give rise to bias whether the measurement are carried out on a sample or on all the units of population. The danger of error is, however, likely to be greater in sampling work, since the units measured are often smaller. Bias may arise due to improper formulation of the decision, problem or wrongly defining the population, specifying the wrong decision, securing an inadequate frame, and so on. Biased observations may result from a poorly designed questionnaire, an ill-trained interviewer, failure of a respondent’s memory, etc. Bias in the flow of data may be due to unorganized collection procedure, faulty editing or coding of responses.

(iii) Bias in Analysis. In addition to bias which arises from faulty process of selection and faulty collection of information, faulty methods of analysis may also introduce bias. Such bias can be avoided by adopting the proper methods of analysis.