average an unknown number of responses per respondent; R [duplicate]

Question

Scenario: I have a df, "scores" of multiple users attempt(s) at passing a test. Each observation is an attempt with the userID, and score. Some users may pass on their first attempt, some might take several; they get unlimited attempts. I want to find the average score for each user.

For example:

userID = c(1:20, sample(1:20, 10, replace = TRUE))
score = c(rnorm(15, mean = 60, sd = 10), rnorm(8, mean = 70, sd = 5), 
rnorm(7, mean = 90, sd = 2))
scores = data.frame(userID, score)

I need an end result data frame that is just a list of unique userIDs with the average of all of their attempts (whether they attempted once or several times).

Of all the dumb approaches I've tried, my most recent was:

avgScores = aggregate(scores, by=list("userID"), "mean")

and got the following error message: "arguments must have same length." I've also tried sorting and sub-setting (actual data frame has time stamps) and wiggling my nose and tapping my shoes together but I'm getting no where and this noob brain is fried.

THANK YOU

Answer 1

Better (more elegant ) here to use aggregate with the formula form :

aggregate(score~userID,scores,mean)

Or using the classic form as you have tried , but you get a slightly different result :

aggregate(scores,by=list(userID),mean) ## using name and not string

Of course if you have big data.frame , better to use one of the solution as suggested in other answers.

Answer 2

#data.table
library(data.table)
DT<-data.table(scores)
DT[,.(mean_score=mean(score)),by=userID]

#dplyr
library(dplyr)
scores %>%
group_by(userID)%>%
summarise(mean_score=mean(score))