Definition
    
                                    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.