  The power of a large clinical trial can be adversely affected by low
recruitment, follow-up and adherence rates. External pilot trials estimate
these rates and use them, via pre-specified decision rules, to determine if the
definitive trial is feasible and should go ahead. There is little
methodological research underpinning how these decision rules, or the sample
size of the pilot, should be chosen. In this paper we propose a hypothesis test
of the feasibility of a definitive trial, to be applied to the external pilot
data and used to make progression decisions. We quantify feasibility by the
power of the planned trial, as a function of recruitment, follow-up and
adherence rates. We use this measure to define hypotheses to test in the pilot,
propose a test statistic, and show how the error rates of this test can be
calculated for the common scenario of a two-arm parallel group definitive trial
with a single normally distributed primary endpoint. We use our method to
re-design TIGA-CUB, an external pilot trial comparing a psychotherapy with
treatment as usual for children with conduct disorders. We then extend our
formulation to include using the pilot data to estimate the standard deviation
of the primary endpoint. and incorporate this into the progression decision.
