Sampling Fundamentals Sampling Sampling may be defined as

Sampling Fundamentals

Sampling • Sampling may be defined as the selection of some part of an aggregate or totality on the basis of which a judgement or inference about the aggregate or totality is made. • In other words, it is the process of obtaining information about an entire population by examining only a part of it.

NEED FOR SAMPLING Save time and money Accurate measurements Only way for infinite population Only choice when a test involves the destruction of the item • Sampling usually enables to estimate the sampling errors and, thus, assists in obtaining information concerning some characteristic of the population. • •

SOME FUNDAMENTAL DEFINITIONS • • Universe/Population Sampling frame Sampling design Statistics and parameters Sampling error Confidence level and significance level Sampling distribution

IMPORTANT SAMPLING DISTRIBUTIONS • • • Sampling distribution of mean Sampling distribution of proportion Student’s t-distribution F distribution Chi-square distribution

CENTRAL LIMIT THEOREM • When sampling is from a normal population, the means of samples drawn from such a population are themselves normally distributed • The significance of the central limit theorem lies in the fact that it permits us to use sample statistics to make inferences about population parameters without knowing anything about the shape of the frequency distribution of that population other than what we can get from the sample.

CONCEPT OF STANDARD ERROR • The standard deviation of sampling distribution of a statistic is known as its standard error (S. E) and is considered the key to sampling theory.

SAMPLE SIZE AND ITS DETERMINATION • • Nature of universe Number of classes proposed Nature of study Type of sampling Standard of accuracy and acceptable confidence level Availability of finance Other considerations: (a) Nature of units, (b) size of the population, (c)size of questionnaire, (d) availability of trained investigators, (e)the conditions under which the sample is being conducted, (f) the time available for completion of the study.