<i>It strikes me that for a condition that is diagnosed in 1 out of 120 children, a population of 300 children is a bit sparse. Is that blasphemy?</i><p>In fact, you don't need large samples to get significant results.  If there is a strong effect, even a very small sample will show it.  If the effect is small, you will need a larger sample.<p>300 is, in fact, a pretty large sample.  I haven't read the full study, but if its a simple random sample, the an effect would have to be quite weak indeed to not show up (in all probability).