Dictionary of Evidence-based Medicine.pdf

Dictionary of Evidence-based Medicine 97
(1993) Managed care. New England Journal of Medicine. 327: 742-7. Weiner },
de Lissovoy G (1993) Razing a tower of Babel: a taxonomy for managed
care and health insurance plans. Journal of Public Health Politics, Policy and
Law. 18: 75-103).
Mann-Whitney statistic
In a comparison of two treatments, the Mann-Whitney statistic, denoted
by P(N>S), is an estimate of the probability that a randomly selected
patient receiving a new treatment (N) would perform better than a randomly
chosen patient receiving the standard treatment (S). For example, if
100 patients on treatment S were switched to the new treatment N and 80
got better and 20 got worse, then P(N>S) is equal to 80/100 or 0.8. Ties are
divided equally to contribute half a patient to each treatment. Calculation
of P(N>S) for results for a matched pair design is undertaken in a similar
manner. Thus if 80 pairs show N to be better, P(N>S) is again 0.8.
Estimates of P(N>S) can be calculated directly from the raw data from
two groups by calculating the proportion of all possible comparisons that
favour N. For example, if n patients received N and s patients received S,
then there are n x s or ns possible comparisons. The same calculation can
be done using data from survival curves.
For continuous data, if we assume that the difference between the score
of a randomly selected patient receiving treatment N and that of a
randomly selected patient receiving treatment S is d, then P(N>S) is an
estimate of the probability that d is greater than zero. To calculate P(N>S),
the difference in the average scores for N and S is calculated and
standardized by dividing by the standard deviation of this difference. The
resulting standard normal score can then be converted to P(N>S) from a
standard normal table. For independent samples, the standard deviation is
calculated as the square root of the sum of the variances of means of scores
for N and S. Thus, if the standard normal score is 0 then P(N>S) = 0.5. If
the standard score is 1.96 then P(N>S) = 0.977 (Colditz GA, Miller JN,
Mosteller F (1989) How study design affects outcome in comparisons of
therapy. I: Medical. Statistics in Medicine. 8: 441-54).
Marginal cost (see under Incremental cost)
Marginal utility
Marginal utility refers to the change in utility resulting from the consumption
of one additional unit of a good.