[R] Usage of p/d/qnorm

ralf.herold@charite.de ralf.herold at charite.de
Wed Jan 12 09:26:13 CET 2000


could You please help: I am looking for a way to formulate test accuracy
measures such as test sensitivity, specificity, predictive values, and
correct classification rate using p/d/qnorm. The tests' primary values
follow a bimodal distribution, which is modelled by a mixture of two normal

     p  * dnorm ((x - u1) / s1) / s1 +
(1 - p) * dnorm ((x - u2) / s2) / s2)

{written like in Venables & Ripley, 2nd Ed., p. 288}. So, the first and
second summand would correspond to truly negative and positive status,
respectively, whereas the test would be termed negative or positive with
respect to a varying x cut off value.

How could one possibly formulate using p/d/qnorm e.g. the specificity:
f(x)= number of positive tests at cut off x / total number of truly
positives ?

Thank You,

Ralf Herold

# Charite Campus Virchow-Klinikum
# Medizinische Fakultaet der Humboldt-Universitaet
# Augustenburger Platz 1, D-13353 Berlin
# ralf.herold at charite.de  pgp welcome

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