Hello R users, I've used the following help two compare two regression line slopes. Wanted to test if they differ significantly:
Hi, I've made a research about how to compare two regression line slopes (of y versus x for 2 groups, "group" being a factor ) using R. I knew the method based on the following statement : t = (b1 - b2) / sb1,b2 where b1 and b2 are the two slope coefficients and sb1,b2 the pooled standard error of the slope (b) which can be calculated in R this way: > df1 <- data.frame(x=1:3, y=1:3+rnorm(3)) > df2 <- data.frame(x=1:3, y=1:3+rnorm(3)) > fit1 <- lm(y~x, df1) > s1 <- summary(fit1)$coefficients > fit2 <- lm(y~x, df2) > s2 <- summary(fit2)$coefficients > db <- (s2[2,1]-s1[2,1]) > sd <- sqrt(s2[2,2]^2+s1[2,2]^2) > df <- (fit1$df.residual+fit2$df.residual) > td <- db/sd > 2*pt(-abs(td), df) My value I get by running this test is :[1] 2.305553e-07 Does it mean the two slopes differ significantly, because this value is in the alpha area, so that I have to reject the null- hypothesis and accept the alternative hypothesis? Is the null-hypothesis: slope1=slope2? Thanks for your help, Benedikt -- ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
