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also have to be available within the clinic to help patients who have difficulty with
self-administration. Finally, staff attitudes and acceptance of patient-reported instru-
ments can make a substantial difference to respondent acceptability.
Interpretability
Interpretability concerns the meaningfulness of scores produced by an instru-
ment. To some extent, the lack of familiarity in the use of instruments may be an
obstacle to interpretation. Three approaches to interpretation have been proposed:
- First, changes in instrument scores have been compared to previously docu-
mented change scores produced by the same instrument at, for example, major
life events such as loss of a job or with modification in the line of management
or lifestyle [ 10 ]. - Secondly, attempts have been made to identify the minimal clinically important
difference (MCID), which is equal to the smallest change in instrument scores
that is perceived as beneficial by patients [ 30 , 31 ]. External judgments, including
summary items such as health transition questions, are used to determine the
MCID. - Thirdly, normative data from the general population can be used to interpret
scores from generic instruments [ 32 , 33 ].
The standardization of instrument scores is an extension of this form of interpre-
tation that allows score changes to be expressed in terms of the score distribution for
the general population and the deviation in this score with particular types of
patients or in particular situations [ 33 ].
Precision
How close to the actual patient experience is the instrument measure or score? It
relates to methods of scaling and scoring items, and the distribution of items over
the range of the construct being measured.
The scaling of items within instruments has important implications for precision.
The binary/dichotomous or “yes” or “no” is the simplest form of response category,
but it does not allow respondents to report different degrees of difficulty or
severity.
The majority of instruments use adjectival or Likert type scales such as strongly
agree, agree, uncertain, disagree, and strongly disagree. Visual analog scales appear
to offer greater precision but there is insufficient evidence to support this and they
may be less acceptable to respondents.
There are a number of instruments that incorporate weighting systems, the most
widely used being preferences or values derived from the general public for utility
measures such as the EuroQol EQ-5D [ 20 ] and the Health Utilities Index [ 34 ].
M. El Gaafary