What is population size:

In simple terms, let’s us say population is the WHOLE SET and

What is sample size: 

let us say it is the SUBSET (which is within the whole set)

The norm

Normally subsets are within the whole set and are smaller than the whole set

Can they be equal ?

Yes , the subset can be equal to the whole set. In that sense, the sample size can be equal to the population size.

Authority:

In a census, data are collected on the entire population, hence the sample size is equal to the population size. (https://en.wikipedia.org/wiki/Sample_size_determination)

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