33instruments can be assessed by methods of construct validation. Factor analysis and
principal component analysis provide empirical support for the dimensionality or
internal construct validity of an instrument [ 46 ]. These statistical techniques can
pick up separate health domains within an instrument [ 47 ].
Responsiveness
Responsiveness is concerned with the measurement of important changes in health
and is therefore relevant when instruments are to be used in an evaluative context
for the measurement of health outcomes. Does the instrument detect changes over
time that matter to patients?
Just as with reliability and validity, estimates of responsiveness are related to
applications within specific populations and are not an inherent or fixed property of
an instrument.
Responsiveness is usually assessed by examining changes in instrument scores
for groups of patients whose health is known to have changed. This may follow an
intervention of known efficacy or a specific life event that is known to affect the
health aspect measured. Alternatively, patients may be asked how their current
health compares to some previous point in time by means of a health transition
question. There is no single agreed upon method of assessing responsiveness and a
number of statistical techniques are used for quantifying responsiveness.
The effect size statistic is equal to the mean change in instrument scores divided
by the baseline standard deviation [ 48 ]. The standardized response mean is equal to
the mean change in scores divided by the standard deviation of the change in scores
[ 49 ]. The modified standardized response mean, sometimes referred to as the index
of responsiveness, is equal to the mean change in scores divided by the standard
deviation of change scores in stable subjects [ 50 ]. The denominator for the latter can
be derived from the test-retest method of reliability testing.
Ideal Properties of PROMs Instrument
Ideal properties of a PROMs instrument can be summarized as follows [ 3 , 51 ]:
• It should be specific to the concept being measured.
• It should be based on end-point model.
• It should have conceptual equivalence (equivalence in relevance and meaning
across languages and cultures).
• It should be based on the conceptual framework.
• It should contain optimum number of items.
• It should have easy and specific measurement properties; i.e., use of the scale
that is the easiest for the intended population to understand.
• It should maintain the confidentiality of the patient.
• It should be reproducible.
2 A Guide to PROMs Methodology and Selection Criteria