  Borrowing of information from historical or external data to inform inference
in a current trial is an expanding field in the era of precision medicine,
where trials are often performed in small patient cohorts for practical or
ethical reasons. Many approaches for borrowing from external data have been
proposed. Even though these methods are mainly based on Bayesian approaches by
incorporating external information into the prior for the current analysis,
frequentist operating characteristics of the analysis strategy are of interest.
In particular, type I error and power at a prespecified point alternative are
in the focus. It is well-known that borrowing from external information may
lead to the alteration of type I error rate. We propose a procedure to
investigate and report the frequentist operating characteristics in this
context. The approach evaluates type I error rate of the test with borrowing
from external data and calibrates the test without borrowing to this type I
error rate. On this basis, a fair comparison of power between the test with and
without borrowing is achieved.

Methodology