> data(anorexia, package="MASS") # weight gain (lbs.) in anorexic women
> attach(anorexia)
> str(anorexia)
'data.frame': 72 obs. of 3 variables:
$ Treat : Factor w/ 3 levels "CBT","Cont","FT": 2 2 2 2 2 2 2 2 2 2 ...
$ Prewt : num 80.7 89.4 91.8 74 78.1 88.3 87.3 75.1 80.6 78.4 ...
$ Postwt: num 80.2 80.1 86.4 86.3 76.1 78.1 75.1 86.7 73.5 84.6 ...
> ft = subset(anorexia, subset=(Treat=="FT")) # just the family therapy threatment
> ft
Treat Prewt Postwt
56 FT 83.8 95.2
57 FT 83.3 94.3
58 FT 86.0 91.5
59 FT 82.5 91.9
60 FT 86.7 100.3
61 FT 79.6 76.7
62 FT 76.9 76.8
63 FT 94.2 101.6
64 FT 73.4 94.9
65 FT 80.5 75.2
66 FT 81.6 77.8
67 FT 82.1 95.5
68 FT 77.6 90.7
69 FT 83.5 92.5
70 FT 89.9 93.8
71 FT 86.0 91.7
72 FT 87.3 98.0
> detach(anorexia)
> rm(anorexia)
> t.test(ft$Prewt, ft$Postwt, paired=T)
Paired t-test
data: ft$Prewt and ft$Postwt
t = -4.1849, df = 16, p-value = 0.0007003
alternative hypothesis: true difference in means is not equal to 0
95 percent confidence interval:
-10.94471 -3.58470
sample estimates:
mean of the differences
-7.264706
>
> t.test(ft$Postwt-ft$Prewt, mu=0, data=ft)
One Sample t-test
data: ft$Postwt - ft$Prewt
t = 4.1849, df = 16, p-value = 0.0007003
alternative hypothesis: true mean is not equal to 0
95 percent confidence interval:
3.58470 10.94471
sample estimates:
mean of x
7.264706
>