Adding qualifiers,
such as "debilitating" or "severe enough to interfere with
your ability to function," or using a quantitative scale
to define eligibility, will identify different samples,
which may vary in the degree of fatigue or its consequences.
The investigator must make some informed guesses about the
type of sample that will provide information that will address
the key aims of the study. Whereas collecting information
about clinically significant fatigue may yield data with
more direct relevance to practice, arbitrarily excluding
patients with milder levels of fatigue may skew the distribution
of some variables and limit the ability of statistical analyses
to identify significant associations. In deciding on the
method for case definition, it is useful to consider the
types of analyses to which the data will ultimately be subjected.