When I collect my sample from different supermarkets, does it make the sampling random?

Question

I want to test for bacteria in lettuce. And I wonder if can get the lettuce from Supermarket A, B, and C and still consider my sample is random.

Answer 1

It depends on what you are collecting, how you select the samples at the supermarket, which supermarkets you are sampling from, and how those particular markets are chosen.

Based on your Additional Information:
I am wondering if you really want "random" or if you want a broad picture of possible contaminants. If random is what you want, pick several markets (probably more than three) over a wide area that may or may not be similar in their lettuce, but put them all their names in a hat a pick randomly, don't just pick 6 different markets.

Picking multiple markets to specifically avoid overlap isn't a random sample, but it will give you a fairly accurate picture of potential contaminants to be found.

Answer 2

It may not be random if the markets get their produce from the same distributor or the same produce grower.