Different results in calculating SD of 2 numbers
-----Original Message----- From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Ron Michael Sent: Wednesday, January 16, 2008 12:15 AM To: r-help at stat.math.ethz.ch Subject: [R] Different results in calculating SD of 2 numbers Hi all, Can anyone tell me why I am getting different results in calculating SD of 2 numbers ?
(1.25-0.95)/2
[1] 0.15
Because the above is not the standard deviation of two numbers. The sample based estimate of the population standard deviation of a set of numbers (which is what sd() computes) is (sum((x-mean(x))^2 )/(length(x)-1))^.5 which for your two numbers would be ( (1.25 - 1.1)^2 + (.95 - 1.1)^2 )^.5
sd(c(1.25, 0.95))
[1] 0.2121320 # why it is different from 0.15? Regards,
Hope this is helpful, Dan Daniel Nordlund Bothell, WA USA