An illustrative and fairly common statistical hypothesis test is the one sample t-test. You use it when you have one sample and you want to test whether that sample likely came from a population by comparing the mean against the known population mean. For this test to work, you have to know the population mean.
In this example, we'll be using R's built-in precip data set that contains precipitation data from 70 US cities.
> head(precip) Mobile Juneau Phoenix Little Rock Los Angeles Sacramento 67.0 54.7 7.0 48.5 14.0 17.2
Don't be fooled by the fact that there are city names in there—this is a regular old vector - it's just that the elements are labeled. We can directly take the mean of this vector, just like a normal one.
> is.vector(precip) [1] TRUE > mean(precip) [1] 34.88571
Let's pretend that we, somehow, know the mean precipitation of the rest of the world—is the US' precipitation significantly...