Let's say I have this data.frame (with 3 variables)
ID Period Score
123 2013 146
123 2014 133
23 2013 150
456 2013 205
456 2014 219
456 2015 140
78 2012 192
78 2013 199
78 2014 133
78 2015 170
Using dplyr I can group them by ID and filter these ID that appear more than once
data <- data %>% group_by(ID) %>% filter(n() > 1)
Now, what I like to achieve is to add a column that is: Difference = Score of Period P - Score of Period P-1 to get something like this:
ID Period Score Difference
123 2013 146
123 2014 133 -13
456 2013 205
456 2014 219 14
456 2015 140 -79
78 2012 192
78 2013 199 7
78 2014 133 -66
78 2015 170 37
It is rather trivial to do this in a spreadsheet, but I have no idea on how I can achieve this in R.
Thanks for any help or guidance.
Here is another solution using lag. Depending on the use case it might be more convenient than diff because the NAs clearly show that a particular value did not have predecessor whereas a 0 using diff might be the result of a) a missing predecessor or of b) the subtraction between two periods.
data %>% group_by(ID) %>% filter(n() > 1) %>%
mutate(
Difference = Score - lag(Score)
)
# ID Period Score Difference
# 1 123 2013 146 NA
# 2 123 2014 133 -13
# 3 456 2013 205 NA
# 4 456 2014 219 14
# 5 456 2015 140 -79
# 6 78 2012 192 NA
# 7 78 2013 199 7
# 8 78 2014 133 -66
# 9 78 2015 170 37