The statistical sampling technique is nowadays generally used in surveys on the particular conditions and problems of the market for several products as well as in opinion surveys. The sampling technique is to supply the "most efficient" results by complying both with the necessity of exact information and of a minimised cost. The latter condition can be realised using the smallest sample size for the requirements of the research. For this purpose, the stratification appears to be a useful means by which the survey's field can be divided in sufficiently homogeneous segments, so as to obtain a smaller sample that the one obtainable with a simple random sample at the same level of statistical significance. More detailed aspects are to be taken into consideration, such as the necessity of attributing to the single strata a non-proportional weight; this situation can also exist after effecting the survey since more interviews can fail in some strata: A non-proportional, i.e. a biased sample, entails the problem of evaluating the reliability of the results obtained, for the purpose of assessing the correct estimate. This particular aspect will be dealt with in this note.