使用分组变量计算值的第95个百分位数 [英] Calculate 95th percentile of values with grouping variable

问题描述

我正在尝试为分水岭分组的多个水质值计算第95个百分位数，例如:

I'm trying to calculate the 95th percentile for multiple water quality values grouped by watershed, for example:

Watershed   WQ
50500101    62.370661
50500101    65.505046
50500101    58.741477
50500105    71.220034
50500105    57.917249

我查看了发布的问题-每个观察变量的百分位数w/r/t分组变量.似乎非常接近我想做的事情，但这是为了每次观察.每个分组变量都需要它.所以很理想，

I reviewed this question posted - Percentile for Each Observation w/r/t Grouping Variable. It seems very close to what I want to do but it's for EACH observation. I need it for each grouping variable. so ideally,

Watershed   WQ - 95th
50500101    x
50500105    y

推荐答案

这可以使用 plyr 库来实现.我们指定分组变量 Watershed ，并要求WQ的95％分位数.

This can be achieved using the plyr library. We specify the grouping variable Watershed and ask for the 95% quantile of WQ.

library(plyr)
#Random seed
set.seed(42)
#Sample data
dat <- data.frame(Watershed = sample(letters[1:2], 100, TRUE), WQ = rnorm(100))
#plyr call
ddply(dat, "Watershed", summarise, WQ95 = quantile(WQ, .95))

和结果

  Watershed     WQ95
    1         a 1.353993
    2         b 1.461711

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