Evaluating Patient-Based Outcome Measures - NIHR Health ...

the relevant condition. This is a problem when
investigators want data from a general sample of
well individuals with which to compare health
status scores of a study sample. This is a common
procedure to provide some form of standard
comparison with which to gauge the health of the
study sample. In the most obvious sense, it is not
possible to ask individuals about the experience of
various problems arising from a condition that they
do not have. A related disadvantage is that diseasespecific
instruments do not allow any obvious or
easy comparison to be made between outcomes of
different treatments for patients with different
health problems. This is only a problem when some
comparative judgement is required of effectiveness
of different treatments for different diseases for
purposes such as resource allocation (Cairns,
1996). Finally disease-specific instruments may not
capture health problems associated with a disease
and its treatment that have not been anticipated.
An instrument with a broader range of items may
be more likely to detect such unexpected effects
(Read, 1993).
Site-specific
In some areas of medicine and surgery, instruments
assessing the impact on the individual of a
disease have come to be considered too broad in
their coverage. Instruments have therefore been
developed that assess health problems in a more
specific part of the body. The Oxford Hip Score
is a 12-item questionnaire designed to be completed
by patients having total hip replacement
surgery (Dawson et al., 1996a). The items are
summed to produce a single score of level of
difficulties arising from the diseased hip. The
Shoulder Disability Questionnaire is a 22-item
questionnaire to assess degree of disability arising
from shoulder symptoms (Croft et al., 1994).
Advantages and disadvantages
The primary intended advantage is that the sitespecific
instrument should contain items that are
particularly relevant to patient groups experiencing
treatment for a very specific region of the
body. They should also be particularly sensitive in
trials of interventions to changes experienced by
patients in that region. For example, a number
of hip scores have been produced because of the
need for outcome measures in orthopaedic
surgery. Differences in outcome between different
arms of a trial of total hip replacement surgery
are quite difficult to detect and questions about
pain due to osteo-arthritis in general may
fail to detect specific problems in the one part
of the body of concern in the evaluation
(Dawson et al., 1996a).
Health Technology Assessment 1998; Vol. 2: No. 14
The principle disadvantage is the consequence
of the relatively narrow focus of such instruments,
namely that such instruments are unlikely to detect
any changes in broader aspects of health or overall
QoL. They are unlikely to be of value in detecting,
for example, unexpected side-effects of an
intervention in a trial.
Dimension specific
Dimension-specific instruments assess one specific
aspect of health status. By far the most common
type of dimension-specific measure is one that
assesses aspects of psychological well-being. An
example is the Beck Depression Inventory (Beck
et al., 1961). It contains 21 items that address
symptoms of depression. The scores for items are
summed to produce a total score. It was largely
developed for use in psychiatric patients but is
increasingly used more widely to assess depression
as an outcome in physically ill populations.
Another commonly assessed dimension of outcome
in trials of physically ill patients is pain
(Cleeland, 1990). The McGill Pain Questionnaire
is an example of a dimension specific instrument
developed for this use in this area (Melzack,
1975). It has several different versions, but the
core of the instrument is formed by a series of
lists of adjectives to describe pain, from each of
which lists the patient selects adjectives that best
describe his or her pain. Individual adjectives are
ranked in terms of severity on the basis of prior
research with patients treated for pain, and the
items chosen by patients are summed to produce
scores for three aspects of pain experience.
Advantages and disadvantages
The principal advantage of such instruments
is that they provide a more detailed assessment
in the area of concern, for example pain or
psychological well-being, than is normally possible
with the short scales usually used in diseasespecific
or generic instruments. Many of the
instruments have been widely used in a range of
clinical populations so that there is a wide range
of comparative data with which to compare
results (Wiklund and Karlberg, 1991). They are
appropriate to medical as well as psychiatric
conditions, although some instruments assessing
psychological well-being need to be slightly
modified either in content or scoring; items
asking about physical problems but intended to
assess somatic aspects of psychological distress
may actually reflect underlying physical disease
(Pincus et al., 1986). This range of instruments
is clearly of particular importance where psychological
well-being is a key concern in a trial.
Many of the other kinds of instruments we are
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