percentiles quantiles with brain data

> brain = read.table(file="D:\\drr07\\stat141\\brain.dat", header = T)
> attach(brain)

> boxplot(brainwt ~ sex) # that "model statement" gives you side-by-side boxplots
> 
> quantile(brainwt[sex == "f"]) # default quartiles
  0%  25%  50%  75% 100% 
 937 1038 1096 1190 1392 
> mean(brainwt[sex == "f"] < 1038) #get proportion corresponding to lower quartile
[1] 0.2467532
> mean(brainwt[sex == "f"] < 1039)
[1] 0.2597403
> mean(brainwt[sex == "f"] <= 1038)
[1] 0.2597403
> # there is a 1038 value in fem brain (see lect 2)
> # 25% data at or below 1038 in sample of 77 fem
> quantile(brainwt[sex == "f"], .3) # find value 30% sample at or below 
 30% 
1045 
> mean(brainwt[sex == "f"] <= 1045) # here's corresponding proportion (percentile)
[1] 0.3116883
> mean(brainwt[sex == "f"] < 1045) # here's corresponding proportion (percentile)
[1] 0.2857143
> # discreteness of relatively small sample
> quantile(brainwt[sex == "f"], .9) # find value 90% sample at or below 
   90% 
1248.2 
> mean(brainwt[sex == "f"] <= 1248.2) # here's corresponding proportion (percentile)
[1] 0.8961039
>