Why do exit polls in India lack credibility?

Since the early 1990s, the conduct of public opinion surveys on various issues has been in vogue. Particularly during election time, such surveys or opinion polls attract considerable attention. Equally important, though hardly surprising, is the fact that they evoke a great deal of controversy.

Assessments about population behaviour based on data from a carefully drawn sample are of course not altogether free of error. There are two distinct types of errors. One is called, in technical jargon, ‘sampling error’ and arises from the sample design itself. The magnitude of this error can be worked out precisely after the sample design is given, and the forecast based on the sample design can be denoted as being subject to this margin of error in either direction.

The other type of error, called non-sampling error, arises from the actual process of collection of data. This category includes errors in observation, measurement and recording. This error cannot be estimated with any degree of precision but can be minimised considerably, though not completely eliminated, by exercising great care in the process of data collection.

Carefully designed opinion surveys have potentially far more of an objective basis than impressionistic reports of journalists in the print medium or political commentators on the electronic media. What should be expected of any serious pollster is that the methodology — the entire gamut of details, including sample design, field work methods and procedures, and analytical procedures by which inferences concerning the population are drawn from sample data — is explicitly and transparently stated.