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The theoretical
(i.e. statistical) analysis of scores depends on comparisons
between obtained scores (or statistics) and expected scores (or
statistics) from the population based on happenstance (chance).
But in practice our comparisons are based almost without exception
on scores obtained from samples, not on populations. For example,
if we record systolic blood pressure in a large number of volunteers
(n=100) and calculate the mean and standard deviation of our sample
scores, we would know on average how far away any particular individual's
score was from the (sample) average. But now if we repeat the
effort (i.e. the measurement of systolic blood pressure in multiple
separate samples of 100 individuals) over and over again (say,
100 times) we would know on average how far away any particular
sample's average score was from the mean of all the (100) samples
tested. The "standard deviation" of the mean of all the sample
means (i.e. the population mean) is the standard error of measurement
(SEM). Scores that fall beyond ± 1 SEMs are interpreted
as unlike most (~67%) of the other scores.
In the
study of dyspnea specifically, and quality of life generally,
efforts to link the standard error of measurement with the minimal
clinically significant difference (MCID) are accelerating. The
MCID is the new metric on the block, designed to facilitate understanding
changes in functional status or health-related quality of life
scores. A minimal clinically significant difference is described
as the smallest difference in a measurable parameter that clinicians
and patients would care about it (e.g. about a 200 ml change
in FEV1). The MCID is a threshold,
in a way, that purportedly represents the change level in a quality
of life instrument where patients begin to notice that there has
been an important improvement or decline (Wyrwich
et al., 2002).
The MCID
is not a fixed quantity but specific for each scale under consideration.
For example, for overall change in asthma-specific quality of
life captured by the Asthma Quality of Life Questionnaire, the
minimal important difference is 0.5 (Juniper
et al., 1994); for overall change in dyspnea captured by the
Transition Dyspnea Index, the minimal important difference is
1 (Witek and Mahler, 2003).
The MCID is neither a simple concept nor a simple calculation
and depends, in part, on physician-generated global transition
ratings.
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