R calculates small numbers, where the result should be zero
This is the result of how any computer program stores numbers, it's not unique to R. A slightly different question is discussed in R FAQ 7.31 but it applies to your situation. You may also enjoy the R Inferno at www.burns-stat.com/pages/Tutor/R_inferno.pdf Bryan *********** Bryan Hanson Professor of Chemistry & Biochemistry DePauw University
On Sep 11, 2012, at 10:23 AM, Andrea Gonnermann <dreli14 at googlemail.com> wrote:
Hi all, my problem is that R does wrong calculations, when the result should be zero. I have the following code: RD_Var1 <- ((0.1*(1-0.1))/100)+((0.2*(1-0.2))/100) RD_Var2 <- ((0.1*(1-0.1))/100)+((0.2*(1-0.2))/100) RD_1 <- 0.1-0.2 RD_1 RD_2 <- 0.1-0.2 FEM_Eff_RD <- ((1/RD_Var1)*RD_1+(1/RD_Var2)*RD_2)/(1/RD_Var1+1/RD_Var2) Q_RD <- 1/RD_Var1*((RD_1-FEM_Eff_RD)^2)+1/RD_Var2*((RD_2-FEM_Eff_RD)^2) Q_RD My result is: 1.540744e-31 When I put the calculated numbers directly into the last formula, I get zero, which is the correct answer: Q_RD <- 1/0.0025*((-0.1--0.1)^2)+1/0.0025*((-0.1--0.1)^2) Why does R calculate the other number instead of zero? Thank you very much for your help!
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