The chisq.test contains following code:

STATISTIC <- sum(sort((x - E)^2/E, decreasing = TRUE))

However, based on book Accuracy and stability of numerical algorithms <

http://ftp.demec.ufpr.br/CFD/bibliografia/Higham_2002_Accuracy%20and%20Stability%20of%20Numerical%20Algorithms.pdf> Table 4.1 on page 89, it is better to sort the data in increasing order than in decreasing order, when the data are non-negative.

A demonstrative example:

x = matrix(c(rep(1.1, 10000)), 10^16, nrow = 10001, ncol = 1) # We have a vector with 10000*1.1 and 1*10^16

c(sum(sort(x, decreasing = TRUE)), sum(sort(x, decreasing = FALSE)))

The result:

10000000000010996 10000000000011000

When we sort the data in the increasing order, we get the correct result. If we sort the data in the decreasing order, we get a result that is off by 4.

Shouldn't the sort be in the increasing order rather than in the decreasing order?

Best regards,

Jan Motl

PS: This post is based on discussion on

https://stackoverflow.com/questions/47847295/why-does-chisq-test-sort-data-in-descending-order-before-summation <

https://stackoverflow.com/questions/47847295/why-does-chisq-test-sort-data-in-descending-order-before-summation>.

[[alternative HTML version deleted]]

______________________________________________

[hidden email] mailing list -- To UNSUBSCRIBE and more, see

https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide

http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.