Is the sample large enough to use a theory-based test?

As mentioned earlier, sand is used in the filter. Different filters had different amounts of sand.

The students split the filters into two groups: those that had more than 2 inches of sand and those that had less. We will explore whether this difference results in different E. coli counts when filtering the water. In this sample, we have results from 20 filters, 14 with more than 2 inches of sand and 6 with less than 2 inches of sand. (See data file Ecoli-sand.)