问题
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$gender=='F'), 3 ), ]
Instead of selecting a number of rows (3 in above case), how can I randomly select 20% of rows with "F"? So of the five rows with "F", how do I randomly sample 20% of those rows.
回答1:
How about this:
mydf[ sample( which(mydf$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
回答2:
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)
回答3:
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")
回答4:
To sample 20%, you can use this to get the sample size:
n = round(0.2 * nrow(mydf[mydf$gender == "F",]))
来源：https://stackoverflow.com/questions/15030910/randomly-sample-a-percentage-of-rows-within-a-data-frame