I have values for
True Positive (TP) and
False Negative (FN) as follows:
TP = 0.25 FN = 0.75
From those values, can we calculate
False Positive (FP) and
True Negative (TN)?
Cross Validated is a question and answer site for people interested in statistics, machine learning, data analysis, data mining, and data visualization. It only takes a minute to sign up.Sign up to join this community
There is quite a bit of terminological confusion in this area. Personally, I always find it useful to come back to a confusion matrix to think about this. In a classification / screening test, you can have four different situations:
Condition: A Not A Test says “A” True positive | False positive ---------------------------------- Test says “Not A” False negative | True negative
In this table, “true positive”, “false negative”, “false positive” and “true negative” are events (or their probability). What you have is therefore probably a true positive rate and a false negative rate. The distinction matters because it emphasizes that both numbers have a numerator and a denominator.
Where things get a bit confusing is that you can find several definitions of “false positive rate” and “false negative rate”, with different denominators.
For example, Wikipedia provides the following definitions (they seem pretty standard):
In all cases, the denominator is the column total. This also gives a cue to their interpretation: The true positive rate is the probability that the test says “A” when the real value is indeed A (i.e., it is a conditional probability, conditioned on A being true). This does not tell you how likely you are to be correct when calling “A” (i.e., the probability of a true positive, conditioned on the test result being “A”).
Assuming the false negative rate is defined in the same way, we then have $FNR = 1 - TPR$ (note that your numbers are consistent with this). We cannot however directly derive the false positive rate from either the true positive or false negative rates because they provide no information on the specificity, i.e., how the test behaves when “not A” is the correct answer. The answer to your question would therefore be “no, it's not possible” because you have no information on the right column of the confusion matrix.
There are however other definitions in the literature. For example, Fleiss (Statistical methods for rates and proportions) offers the following:
(He also acknowledges the previous definitions but considers them “wasteful of precious terminology”, precisely because they have a straightforward relationship with sensitivity and specificity.)
Referring to the confusion matrix, it means that $FPR = FP / (TP + FP)$ and $FNR = FN / (TN + FN)$ so the denominators are the row totals. Importantly, under these definitions, the false positive and false negative rates cannot directly be derived from the sensitivity and specificity of the test. You also need to know the prevalence (i.e., how frequent A is in the population of interest).
Fleiss does not use or define the phrases “true negative rate” or the “true positive rate” but if we assume those are also conditional probabilities given a particular test result / classification, then @guill11aume answer is the correct one.
In any case, you need to be careful with the definitions because there is no indisputable answer to your question.
EDIT: see the answer of Gaël Laurans, which is more accurate.
If your true positive rate is 0.25 it means that every time you call a positive, you have a probability of 0.75 of being wrong. This is your false positive rate. Similarly, every time you call a negative, you have a probability of 0.25 of being right, which is your true negative rate.
None if this makes any sense if "positive" and "negative" do not make sense for the problem at hand. I see many problems where "positive" and "negative" are arbitrary forced choices on an ordinal or continuous variable. FP, TP, sens, spec are only useful for all-or-nothing phenomena.
1) True +ve and false -ve make 100% 2) False +ve and true -ve make 100% 3) There is no relation between true positives and false positives.
We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.
Thank you for your interest in this question.
Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
Would you like to answer one of these unanswered questions instead?