[R] problem with sum function

Hi! I'm running R version 2.13.0 (2011-04-13) Platform: i386-pc-mingw32/i386 (32-bit) When i type in the command: sum(c(-0.2, 0.8, 0.8, -3.2, 1.8)) R returns the value: -5.551115e-17 Why doesn't R return zero in this case? There shouldn't be any rounding error in a simple sum. Thanks, M

[R] estimating treatment effect in blocked experiment

R-help, I am interested in estimating the effect of a treatment (2 levels) on a response. I've used a randomized blocked experiment (5 blocks). I run the full model, let's say that it is... lm1 <- lm(resp ~ treat + block) ...and find that there are no significant block effects. Now with