Amy Mikhail wrote:
Dear Peter,
Many thanks for your reply. Unfortunately I don't have access to the
reference book.
However it looks to me like the gold standard should be in the rows and
the comparison method should be in the columns for the test to give the
right answer (since the location of the gold standard determines which
are true and false positives, etc, which determines what the
denominators of the formulae are).
I tried swapping around the negatives and positives, but that doesn't
make much difference to the result (it is still the wrong way around):
> t1
positive negative
positive 201 1
negative 0 18
> sensSpec(t1)
Simple Sensitivity and Specitivity Output
Input Matrix:
positive negative
positive 201 1
negative 0 18
The sample of sensitivity is: 100%
The sample of specificity is: 94.7%
Umm, maybe I'm not quite awake yet, but if you're switching positive and
negatives, the 1 should end up in the other corner so that the gold
standard has 202 positives and 18 negatives, with 1 false positive and
no false negatives. I.e., the sensitivity ia 201/202 and specificity is
18/18. Transposing the tables would give you predictive probabilities,
which are different beasts entirely.
....
sensSpec(t1)
Simple Sensitivity and Specitivity Output
Input Matrix:
ref
r1a negative positive
negative 18 1
positive 0 201
The sample of sensitivity is: 100%
The sample of specificity is: 99.5%
--
O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B
c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalga...@biostat.ku.dk) FAX: (+45) 35327907
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