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Subject:
From:
Cynthia Good Mojab <[log in to unmask]>
Reply To:
Lactation Information and Discussion <[log in to unmask]>
Date:
Thu, 13 Feb 2003 12:32:25 -0800
Content-Type:
text/plain
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"Well, if 50% are false-positive, then the other 50% would be false
negative, no?"

No. There are four types of results that can occur when a test is
conducted:

* False positives occur when a test indicates that a person has the
condition in question when they really do not. (High false positive rates
are undesirable; If people are unaware that a test has a high false
positive rate, it can result in many people taking action to treat a
condition that they do not actually have.)
* True positives occur when a test indicates that a person has the
condition in question when they really do. (High true positives make the
test useful.)
* False negatives occur when a test indicates that a person does not have
the condition in question when they really do. (High false negative rates
are undesirable; if people are unaware that a test has a high false
negative rate, it can result in many people not taking action to treat a
condition that they actually have.)
* True negatives occur when a test indicates that a person does not have
the condition in question when they really do not. (High true negatives
make the test useful.)

The proportion of false positives plus the proportion of true positives
equals 100% of the positive results.
The proportion of false negatives plus the proportion of true negatives
equals 100% of the negative results.

So a 50% false-positive rate only means that, *of tests results that were
positive*, 50% of the results correctly identified the existence of the
condition and 50% of the results indicated the condition existed when it
did not. A 50% false-positive rate doesn't tell us anything about what
proportion of negative test results are correct and what proportion are not
correct.

With my eyes glazing over trying to type this post without any errors,

Cynthia

Cynthia Good Mojab, MS clinical psychology, IBCLC, RLC
Ammawell
Email: [log in to unmask]
Web site: http://home.attbi.com/~ammawell

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