3.12homework

2

library(MASS)

## 
## Attaching package: 'MASS'

## The following object is masked from 'package:dplyr':
## 
##     select

head(dta <- cats)

##   Sex Bwt Hwt
## 1   F 2.0 7.0
## 2   F 2.0 7.4
## 3   F 2.0 9.5
## 4   F 2.1 7.2
## 5   F 2.1 7.3
## 6   F 2.1 7.6

coef(lm(Bwt ~ Sex, data = cats))

## (Intercept)        SexM 
##   2.3595745   0.5404255

coef(lm(Bwt ~ Sex, data = cats, contrasts = list(Sex = "contr.sum")))

## (Intercept)        Sex1 
##   2.6297872  -0.2702128

因為虛擬變數的關係， 上者截距為F(參照組)之平均，迴歸係數為M(操弄組)平均減截距。 下者截距為未加權總平均數，迴歸係數為截距減去M(操弄組)的細格平均數。