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
```

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
```

thanks

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))
```

and the results

```
Watershed WQ95
1 a 1.353993
2 b 1.461711
```

### Answer：

I hope I understand your question correctly. Is this what you’re looking for?

```
my.df <- data.frame(group = gl(3, 5), var = runif(15))
aggregate(my.df$var, by = list(my.df$group), FUN = function(x) quantile(x, probs = 0.95))
Group.1 x
1 1 0.6913747
2 2 0.8067847
3 3 0.9643744
```

*EDIT*

Based on Vincent’s answer,

```
aggregate(my.df$var, by = list(my.df$group), FUN = quantile, probs = 0.95)
```

also works (you can skin a cat 1001 ways – I’ve been told). A side note, you can specify a vector of desired -iles, say `c(0.1, 0.2, 0.3...)`

for deciles. Or you can try function `summary`

for some predefined statistics.

```
aggregate(my.df$var, by = list(my.df$group), FUN = summary)
```

### Answer：

Use a combination of the tapply and quantile functions. For example, if your dataset looks like this:

```
DF <- data.frame('watershed'=sample(c('a','b','c','d'), 1000, replace=T), wq=rnorm(1000))
```

Use this:

```
with(DF, tapply(wq, watershed, quantile, probs=0.95))
```

### Answer：

In Excel, you’re going to want to use an array formula to make this easy. I suggest the following:

```
{=PERCENTILE(IF($A2:$A6 = Watershed ID, $B$2:$B$6), 0.95)}
```

Column A would be the Watershed ids, and Column B would be the WQ values.

Also, be sure to enter the formula as an array formula. Do so by pressing Ctrl+Shift+Enter when entering the formula.

### Answer：

Using the data.table-package you can do:

```
set.seed(42)
#Sample data
dt <- data.table(Watershed = sample(letters[1:2], 100, TRUE), WQ = rnorm(100))
dt[ ,
j = .(WQ95 = quantile(WQ, .95, na.rm = TRUE),
by = Watershed]
```

Tags: excelexcel