Did the groups receive equal treatment?
Watch for differences in care other than the study under treatment. These
are called "co-interventions" and may not be mentioned.
Application to Pollock et al's
thyroxine paper
In this study, there may have been some co-interventions in the background.
There are none mentioned explicitly and with larger numbers, one could
assume that these would be uniformly dispersed amoungst the patients.