Bug 16704 - function cor.test doesn't work for small values when the p-value is smaller than 2.2x10^-16 and the calculation of p-value is wrong for small p-values close to 2.2x10^-16
Summary: function cor.test doesn't work for small values when the p-value is smaller t...
Alias: None
Product: R
Classification: Unclassified
Component: Accuracy (show other bugs)
Version: R 3.2.2
Hardware: All All
: P5 critical
Assignee: R-core
Depends on:
Reported: 2016-02-09 00:13 UTC by Chuan Li
Modified: 2016-02-09 14:59 UTC (History)
1 user (show)

See Also:


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Description Chuan Li 2016-02-09 00:13:42 UTC
# moreover if you look at the numbers closely,
a <- c(0.998, 2.022, 3.011, 3.993, 5.006, 5.983, 6.994, 8.017, 8.995, 9.984)
b <- 1:10
k1 <- cor.test(a, b, method="pearson") 
k1$p.value # value is 0
k2 <- cor.test(a, -b, method="pearson") 
k2$p.value # p value is 1.7xe-20
## The two numbers should be the sample

a <- c(1.059,1.980,2.932,4.042,5.015,5.892,7.019,7.954,8.990,10.032)
b <- 1:10
k1 <- cor.test(a, b, method="pearson") 
k1$p.value # 2.220446e-15
k2 <- cor.test(a, -b, method="pearson") 
k2$p.value # 2.272542e-15

# The correct value should be
pt(k1$statistic, k1$parameter, lower.tail = F)*2   # 2.272542e-15
pt(k2$statistic, k2$parameter, lower.tail = T)*2   # 2.272542e-15

# Therefore, when the correlation is positive, the calculation has an incorrect lower bound and the calculation is not correct.
# Possible reason: 2.2e-16 is the value of .Machine$double.eps - the smallest floating point number such that 1 + x != 1
# Is it because in your code somewhere you use 1-pvalue?
(1-pt(k2$statistic, k2$parameter, lower.tail = F))*2   # 2.220446e-15
# which is the p-value calculated by the current function cor.test

# Thank you for your attention.
# Best wishes!
# any problem? Feel free to contact lichuan@umich.edu
Comment 1 Martin Maechler 2016-02-09 08:25:55 UTC
indeed, the small accuracy losses happen because  1 - pt() or  1 - pnorm() 
are used instead of  p*(.., lower.tail=FALSE).

I'll commit the fixes within a day or so.