Generalizing: Sample Size and Variety

Rule #1: The sample should be sufficiently numerous and various.

A single instance is usually not enough to support a generalization. In the absence of other knowledge, the fact that a single S is P won't tell us whether S and P are connected, so generalizing that all S are P would be hasty. We need to look at a number of Ss; if all of them are P, then we have better evidence of a connection.

Also, it's equally important to test a variety of Ss. How much variety is enough? The general rule is that a sample of Ss should vary in every property (other than being S) that might be responsible for their being P.


Suppose you were considering buying a Toyota. You might ask people who owned Toyotas whether they were satisfied. Suppose all of them have had problems with the car. Does that indicate that Toyotas are unreliable? This conclusion would be stronger if you checked cars of different years, with different options, bought from different dealers.

Sample size and variety | Disconfirming instances |
Sensible links between S and P

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