Using R, Randomly Assigning Students Into Groups Of 4

Question

I'm still learning R and have been given the task of grouping a long list of students into groups of four based on another variable. I have loaded the data into R as a data frame. How do I sample entire rows without replacement, one from each of 4 levels of a variable and have R output the data into a spreadsheet?

So far I have been tinkering with a for loop and the sample function but I'm quickly getting over my head. Any suggestions? Here is sample of what I'm attempting to do. Given:

Last.Name <- c("Picard","Troi","Riker","La Forge", "Yar", "Crusher", "Crusher", "Data")
First.Name <- c("Jean-Luc", "Deanna", "William", "Geordi", "Tasha", "Beverly", "Wesley", "Data")
Email <- c("a@a.com","b@b.com", "c@c.com", "d@d.com", "e@e.com", "f@f.com", "g@g.com", "h@h.com")
Section <- c(1,1,2,2,3,3,4,4)

df <- data.frame(Last.Name,First.Name,Email,Section)

I want to randomly select a Star Trek character from each section and end up with 2 groups of 4. I would want the entire row's worth of information to make it over to a new data frame containing all groups with their corresponding group number.

A quick Google search for `sample R` returns this: http://stat.ethz.ch/R-manual/R-devel/library/base/html/sample.html — tcash21, Jan 15 '15 at 00:11
@tcash21 - that's not very helpful. OP already mentions the `sample` function. Jeremy - can you provide a simplified example of your data and expected output to improve this question? — thelatemail, Jan 15 '15 at 00:13
whoops didn't see that he mentioned the sample function. Yes a reproducible example is required though. — tcash21, Jan 15 '15 at 00:16
I've made an edit showing a simplified example of my task. Thank you so much for the quick response and the help so far. — JForsythe, Jan 15 '15 at 01:02

score 6 · Accepted Answer · answered Jan 15 '15 at 02:09

I'd use the wonderful package 'dplyr'

require(dplyr)

random_4 <- df %>% group_by(Section) %>% slice(sample(c(1,2),1))

random_4
Source: local data frame [4 x 4]
Groups: Section

  Last.Name First.Name   Email Section
1      Troi     Deanna b@b.com       1
2  La Forge     Geordi d@d.com       2
3   Crusher    Beverly f@f.com       3
4      Data       Data h@h.com       4

random_4
Source: local data frame [4 x 4]
Groups: Section

  Last.Name First.Name   Email Section
1    Picard   Jean-Luc a@a.com       1
2     Riker    William c@c.com       2
3   Crusher    Beverly f@f.com       3
4      Data       Data h@h.com       4

%>% means 'and then'

The code is read as:

Take DF AND THEN for all 'Section', select by position (slice) 1 or 2. Voila.

Wow. That's really awesome. I'm downloading that package right now. Makes for a lot prettier of an outcome then what I was doing. Thanks! — JForsythe, Jan 15 '15 at 02:17

score 2 · Answer 2 · answered Jan 15 '15 at 01:42

I suppose you have 8 students: First.Name <- c("Jean-Luc", "Deanna", "William", "Geordi", "Tasha", "Beverly", "Wesley", "Data").

If you wish to randomly assign a section number to the 8 students, and assuming you would like each section to have 2 students, then you can either permute Section <- c(1, 1, 2, 2, 3, 3, 4, 4) or permute the list of the students.

First approach, permute the sections:

> assigned_section <- print(sample(Section))
[1] 1 4 3 2 2 3 4 1

Then the following data frame gives the assignments:

assigned_students <- data.frame(First.Name, assigned_section)

Second approach, permute the students:

> assigned_students <- print(sample(First.Name))
[1] "Data"     "Geordi"   "Tasha"    "William"  "Deanna"   "Beverly"  "Jean-Luc" "Wesley"

Then, the following data frame gives the assignments:

assigned_students <- data.frame(assigned_students, Section)

JForsythe · Answer 3 · 2015-01-15T02:51:15.237

Alex, Thank You. Your answer wasn't exactly what I was looking for, but it inspired the correct one for me. I had been thinking about the process from a far too complicated point of view. Instead of having R select rows and put them into a new data frame, I decided to have R assign a random number to each of the students and then sort the data frame by the number:

First, I broke up the data frame into sections:

df1<- subset(df, Section ==1)

df2<- subset(df, Section ==2)

df3<- subset(df, Section ==3)

df4<- subset(df, Section ==4)

Then I randomly generated a group number 1 through 4.

Groupnumber <-sample(1:4,4, replace=F)

Next, I told R to bind the columns:

Assigned1 <- cbind(df1,Groupnumber)

*Ran the group number generator and cbind in alternating order until I got through the whole set. (Wanted to make sure the order of the numbers was unique for each section).

Finally row binding the data set back together:

Final_List<-rbind(Assigned1,Assigned2,Assigned3,Assigned4)

Thank you everyone who looked this over. I am new to data science, R, and stackoverflow, but as I learn more I hope to return the favor.

score 1 · Answer 4 · answered Jun 20 '17 at 11:44

I'd suggest the randomizr package to "block assign" according to section. The block_ra function lets you do this in a easy-to-read one-liner.

install.packages("randomizr")
library(randomizr)

df$group <- block_ra(block_var = df$Section, 
             condition_names = c("group_1", "group_2"))

You can inspect the resulting sets in a variety of ways. Here's with base r subsetting:

df[df$group == "group_1",]

  Last.Name First.Name   Email Section   group
2      Troi     Deanna b@b.com       1 group_1
3     Riker    William c@c.com       2 group_1
6   Crusher    Beverly f@f.com       3 group_1
7   Crusher     Wesley g@g.com       4 group_1

df[df$group == "group_2",]

  Last.Name First.Name   Email Section   group
1    Picard   Jean-Luc a@a.com       1 group_2
4  La Forge     Geordi d@d.com       2 group_2
5       Yar      Tasha e@e.com       3 group_2
8      Data       Data h@h.com       4 group_2

score 0 · Answer 5 · answered Jan 15 '15 at 05:13

0

If you want to roll your own:

set <-  tapply(1:nrow(df), df$Section, FUN = sample, size = 1)
df[set,]    # show the sampled set
df[-set,]   # show the complimentary set

answered Jan 15 '15 at 05:13

Rick

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Using R, Randomly Assigning Students Into Groups Of 4

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