Both of these refer to the selection of less than 100% of the items comprising a population where the objective is to draw a conclusion about the characteristics of the population.
Selection of items may be done using either statistical or non-statistical techniques.
Statistical techniques are called sampling. These techniques require
(i) the units in the population to have the same chance of being selected(ii) the items for examination to be selected randomly and (iii) the conclusion to be drawn using tables or formulas based on probability theory. The sample must be large enough to provide statistically meaningful results.
Test checking is non statistical technique. In test checking the auditor picks up the items for examination on the basis of nature, size and materiality, Test checking involves judgment.
To be true representative, selection of items should be free from any bias.