Professor Torgerson's discussions of randomised trials suggest that a large sample is needed to produce a statistically and educationally significant result - she proposes a rather arbitrary criterion of half a standard deviation's improvement as significant. Prof Debra Myhill’s trial on English grammar cost £750k, which, in our case, we have not got.
But – if we collect evidence from classes taking part in research, and have a realistic comparison group, we can then analyse it in various ways, including random selection, say, of two pupils per group in the middle, upper and lower bands. Differences in a small sample might not reach statistical significance, but they could form part of a wider analysis, for example by confirming the results obtained by a whole class. It seems, incidentally, strange to argue that a sample might provide better evidence than a whole group (The term cohort belongs to Roman military history rather than twenty-first century education).
Silver, rather than gold, standard research, but within our means.
Further notes on the design of small scale research are here.