  Experiments are a commonly used method of research in software engineering
(SE). Researchers report their experiments following detailed guidelines.
However, researchers do not, in the field of test-driven development (TDD) at
least, specify how they operationalized the response variables and the
measurement process. This article has three aims: (i) identify the response
variable operationalization components in TDD experiments that study external
quality; (ii) study their influence on the experimental results;(ii) determine
if the experiment reports describe the measurement process components that have
an impact on the results. Sequential mixed method. The first part of the
research adopts a quantitative approach applying a statistical an\'alisis (SA)
of the impact of the operationalization components on the experimental results.
The second part follows on with a qualitative approach applying a systematic
mapping study (SMS). The test suites, intervention types and measurers have an
influence on the measurements and results of the SA of TDD experiments in SE.
The test suites have a major impact on both the measurements and the results of
the experiments. The intervention type has less impact on the results than on
the measurements. While the measurers have an impact on the measurements, this
is not transferred to the experimental results. On the other hand, the results
of our SMS confirm that TDD experiments do not usually report either the test
suites, the test case generation method, or the details of how external quality
was measured. A measurement protocol should be used to assure that the
measurements made by different measurers are similar. It is necessary to report
the test cases, the experimental task and the intervention type in order to be
able to reproduce the measurements and SA, as well as to replicate experiments
and build dependable families of experiments.
