# 直線迴歸 (General linear model)
###### tags: `R教學` `基礎統計`

rm(list=ls())

library("multcomp")

dat <- iris[,c(1,2,5)]

#### 繪圖 ####
tiff(filename = "lm.tif",
     width = 350*3.5, height = 240*3.5, pointsize = 20)
plot(Sepal.Width ~ Sepal.Length, data = dat, las = 1)
abline(lm(Sepal.Width ~ Sepal.Length, 
          data = dat),
       lwd = 3)
points(Sepal.Width ~ Sepal.Length, data = dat, col = Species, pch = 19)

abline(lm(Sepal.Width ~ Sepal.Length, 
          data = subset(dat, Species == levels(dat$Species)[1])), 
       col = 1, lty = 2)
abline(lm(Sepal.Width ~ Sepal.Length, 
          data = subset(dat, Species == levels(dat$Species)[2])), 
       col = 2, lty = 2)
abline(lm(Sepal.Width ~ Sepal.Length, 
          data = subset(dat, Species == levels(dat$Species)[3])), 
       col = 3, lty = 2)

legend("topleft",legend=levels(dat$Species), 
       pch = 19, col=1:3, lty=2, yjust=0, inset = 0.01)
dev.off()

#### 統計模型 ####
lm.model <- lm(Sepal.Width ~ Sepal.Length + Species, data = dat)
summary(lm.model)
anova(lm.model)
summary(Posthoc <- glht(lm.model, linfct = mcp(Species = "Tukey")))
(letter.dat <- cld(Posthoc,
                  alpha   = 0.05,
                  Letters = letters,
                  decreasing = TRUE))