CODEWITHSUNDEEP
rrowsubsetrandom
ATMathew
ATMathew

Reputation: 12866

Randomly sample a percentage of rows within a data frame

Related to this question.

gender <- c("F", "M", "M", "F", "F", "M", "F", "F")
age    <- c(23, 25, 27, 29, 31, 33, 35, 37)
mydf <- data.frame(gender, age) 

mydf[ sample( which(mydf$gender=='F'), 3 ), ]

Instead of selecting a number of rows (3 in above case), how can I randomly select 20% of rows with "F"? So of the five rows with "F", how do I randomly sample 20% of those rows.

Upvotes: 23

Views: 40333

Answers (4)

Zhen Liang
Zhen Liang

Reputation: 361

You can use sample_frac() function in dplyr package.

e.g. If you want to sample 20 % within each group:

mydf %>% sample_frac(.2)

If you want to sample 20 % within each gender group:

mydf %>% group_by(gender) %>% sample_frac(.2)

Upvotes: 23

A5C1D2H2I1M1N2O1R2T1
A5C1D2H2I1M1N2O1R2T1

Reputation: 193677

Self-promotion alert. I wrote a function that allows convenient stratified sampling, and I've included an option to subset levels from the grouping variables before sampling.

The function is called stratified and can be used in the following ways:

set.seed(1)
# Proportional sample
stratified(mydf, group="gender", size=.2, select=list(gender = "F"))
#   gender age
# 4      F  29
# Fixed-size sampling
stratified(mydf, group="gender", size=2, select=list(gender = "F"))
#   gender age
# 4      F  29
# 5      F  31

You can specify multiple groups (for example if your data frame included a "state" variable and you wanted to group by "state" and "gender" you would specify group = c("state", "gender")). You can also specify multiple "select" arguments (for example, if you wanted only female respondents from California and Texas, and your "state" variable used two-letter state abbreviations, you could specify select = list(gender = "F", state = c("CA", "TX"))).

The function itself can be found here or you can download and install the package (which gives you convenient access to the help pages and examples) by using install_github from the "devtools" package as follows:

# install.packages("devtools")
library(devtools)
install_github("mrdwabmisc", "mrdwab")

Upvotes: 3

Paul Hiemstra
Paul Hiemstra

Reputation: 60984

To sample 20%, you can use this to get the sample size:

n = round(0.2 * nrow(mydf[mydf$gender == "F",]))

Upvotes: 2

Ben
Ben

Reputation: 42343

How about this:

mydf[ sample( which(mydf$gender=='F'), round(0.2*length(which(mydf$gender=='F')))), ]

Where 0.2 is your 20% and length(which(mydf$gender=='F')) is the total number of rows with F

Upvotes: 15

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