R: splitting dataset into quartiles/deciles. What is the right method? [duplicate]

Question

I am very new with R, so hoping I can get some pointers on how to achieve the desired manipulation of my data.

I have an array of data with three variables.

  gene_id       fpkm  meth_val
1 100629094     0.000 0.0063
2 100628995     0.000 0.0000
3 102655614   111.406 0.0021

I'd like to plot the average meth_val after stratifying my gene_ids based on fpkm into quartiles or deciles.

Once I load my data into a dataframe...

data <- read.delim("myfile.tsv", sep='\t')

I can determine the fpkm deciles using:

quantile(data$fpkm, prob = seq(0, 1, length = 11), type = 5

which yields

          0%          10%          20%          30%          40%          50%
0.000000e+00 9.783032e-01 7.566164e+00 3.667630e+01 1.379986e+02 3.076280e+02
         60%          70%          80%          90%         100%
5.470552e+02 8.875592e+02 1.486200e+03 2.974264e+03 1.958740e+05

From there, I'd like to essentially split the dataframe into 10 groups based on whether the fpkm_val fits into one of these deciles. Then I'd like to plot the meth_val of each decile in ggplot as a box plot and perform a statistical test across deciles.

The main thing I'm really stuck on is how to split my dataset in the proper way. Any assistance would be hugely appreciated!

Thanks a bunch!

Answer 1

Another way would be ntile() in dplyr.

library(tidyverse)

foo <- data.frame(a = 1:100,
                  b = runif(100, 50, 200),
                  stringsAsFactors = FALSE)

foo %>%
    mutate(quantile = ntile(b, 10))

#  a         b quantile
#1 1  93.94754        2
#2 2 172.51323        8
#3 3  99.79261        3
#4 4  81.55288        2
#5 5 116.59942        5
#6 6 128.75947        6

Answer 2

Perhaps easier like this:

data$qunatil = cut( data$fpkm, quantile(data$fpkm, prob = seq(0, 1, length = 11), type = 5) )

Answer 3

you can try using Hmisc library and cut2 function. You can cut vector into different groups by stating the cutpoints. Here is an example:

library(Hmisc)
data <- data.frame(gene_id=sample(c("A","B","D", 100), 100, replace=TRUE),
               fpkm=abs(rnorm(100, 100, 10)),
               meth_val=abs(rnorm(100, 10, 1)))
quantiles <- quantile(data$fpkm, prob = seq(0, 1, length = 11), type = 5)
data$cutted <- cut2(data$fpkm, cuts = as.numeric(quantiles))

And you will get the same data frame with additional columns for split:

    gene_id      fpkm  meth_val        cutted
1         B 102.16511  8.477469 [100.4,103.2)
2         A 110.59269  9.256172 [106.4,110.9)
3         B  93.15691 10.560936 [ 92.9, 95.3)
4         B 105.74879 10.301358 [103.2,106.4)
5         A  96.12755 11.336484 [ 95.3, 96.8)
6         B 106.29204  8.286120 [103.2,106.4)
...

Moreover you can cut using cut2 specifying by quantiles groups too. Read more ?cut2.