An important feature of the NSS sampling design is that the total sample of first stage units is drawn in the form of two or more independent and parallel samples, termed as interpenetrating sub-samples. Each sub- sample is drawn by the same
sampling scheme and is capable of providing valid estimates of the population parameters. The comparison of sub-sample wise estimates shows the margin of uncertainty associated with the combined sample estimate.

Interpenetrating sub-samples have been used in NSS (i) to obtain valid estimates from each sub-round (season) of the survey round, and (ii) to ensure that Central and State samples for any State/ UT cover independent and equally valid samples of units.

The samples surveyed by the NSSO staff are termed as Central sample and the matched samples surveyed by State Government staff are termed as State sample.