Lee_Kennedy - 1 year ago 172

R Question

I am wanting to apply the Grubbs test to a set of data repeatedly until it ceases to find outliers. I want the outliers flagged rather than removed so that I can plot the data as a histogram with the outliers a different colour. I have used grubbs.test from the outliers package to manually identify outliers but cannot figure out how to cycle through them and flag them successfully. The sort of output I am aiming for is like the following:

`X Outlier`

152.36 Yes

130.38 Yes

101.54 No

96.26 No

88.03 No

85.66 No

83.62 No

76.53 No

74.36 No

73.87 No

73.36 No

73.35 No

68.26 No

65.25 No

63.68 No

63.05 No

57.53 No

Answer Source

Looks like you would need a short function to do what you want:

```
library(outliers)
library(ggplot2)
X <- c(152.36,130.38,101.54,96.26,88.03,85.66,83.62,76.53,
74.36,73.87,73.36,73.35,68.26,65.25,63.68,63.05,57.53)
grubbs.flag <- function(x) {
outliers <- NULL
test <- x
grubbs.result <- grubbs.test(test)
pv <- grubbs.result$p.value
while(pv < 0.05) {
outliers <- c(outliers,as.numeric(strsplit(grubbs.result$alternative," ")[[1]][3]))
test <- x[!x %in% outliers]
grubbs.result <- grubbs.test(test)
pv <- grubbs.result$p.value
}
return(data.frame(X=x,Outlier=(x %in% outliers)))
}
```

Here's the output:

```
grubbs.flag(X)
X Outlier
1 152.36 TRUE
2 130.38 TRUE
3 101.54 FALSE
4 96.26 FALSE
5 88.03 FALSE
6 85.66 FALSE
7 83.62 FALSE
8 76.53 FALSE
9 74.36 FALSE
10 73.87 FALSE
11 73.36 FALSE
12 73.35 FALSE
13 68.26 FALSE
14 65.25 FALSE
15 63.68 FALSE
16 63.05 FALSE
17 57.53 FALSE
```

And if you want a histogram with different colors, you can use the following:

```
ggplot(grubbs.flag(X),aes(x=X,color=Outlier,fill=Outlier))+
geom_histogram(binwidth=diff(range(X))/30)+
theme_bw()
```