已知婴儿出生体重随种族的不同而不同。白种人婴儿的出生体重比其他种族的重。出生体重也随孕期的增长而增加，足月（40周）的婴儿通常比不足月（小于40周）的重。比较不同种族婴儿出生体重，作协方差分析。
孕期（days）单位为天，出生体重（weight）单位为盎司。
baby <- rep(c("W", "B", "S", "A"), each = 6)
days <- c(260, 275, 278, 280, 282, 288, 260, 263, 270, 278, 281, 285, 262, 264,
270, 275, 280, 284, 260, 271, 274, 279, 281, 283)
weight <- c(130, 135, 138, 142, 146, 149, 115, 118, 120, 125, 128, 132, 113,
115, 120, 121, 127, 132, 111, 124, 117, 118, 120, 122)
data.baby <- data.frame(baby, days, weight)
library(HH)
## Loading required package: lattice
## Loading required package: grid
## Loading required package: latticeExtra
## Loading required package: RColorBrewer
## Loading required package: multcomp
## Loading required package: mvtnorm
## Loading required package: survival
## Loading required package: splines
## Loading required package: TH.data
ancova(weight ~ days + baby, data = data.baby)
## Analysis of Variance Table
##
## Response: weight
## Df Sum Sq Mean Sq F value Pr(>F)
## days 1 1055 1055 138.3 3.6e-10 ***
## baby 3 1300 433 56.8 1.1e-09 ***
## Residuals 19 145 8
## ---
## Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
ancova(weight ~ days * baby, data = data.baby)
## Analysis of Variance Table
##
## Response: weight
## Df Sum Sq Mean Sq F value Pr(>F)
## days 1 1055 1055 161.85 8.8e-10 ***
## baby 3 1300 433 66.46 3.0e-09 ***
## days:baby 3 41 14 2.08 0.14
## Residuals 16 104 7
## ---
## Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
plot(days[baby == "W"], weight[baby == "W"], xlim = c(260, 290), ylim = c(110,
150), xlab = "days", ylab = "weight", pch = 15, col = "green")
points(days[baby == "B"], weight[baby == "B"], pch = 15, col = "red")
points(days[baby == "S"], weight[baby == "S"], pch = 15, col = "blue")
points(days[baby == "A"], weight[baby == "A"], pch = 15, col = "yellow")
legend(260, 150, c("White", "Black", "Spanish", "Asian"), col = c("green", "red",
"blue", "yellow"), pch = c(15, 15, 15, 15))
结果表明，孕期和种族对婴儿出生体重大小都有很大影响。随着孕期时间的增加，各个种族婴儿出生体重均增加；不同种族对于婴儿出生体重影响较大。