To put it another way, statistical analysis with small samples is like making astronomical observations with binoculars. You are limited to seeing big things: planets, stars, moons and the occasional comet. But just because you don’t have access to a high-powered telescope doesn’t mean you cannot conduct astronomy. Galileo, in fact, discovered Jupiter’s moons with a telescope with the same power as many of today’s binoculars.
Just as with statistics, just because you don’t have a large sample size doesn’t mean you cannot use statistics. Again, the key limitation is that you are limited to detecting large differences between designs or measures.
Fortunately, in user-experience research we are often most concerned about these big differences—differences users are likely to notice, such as changes in the navigation structure or the improvement of a search results page.