Select first observed data and utilize mutate

Question

I am running into an issue with my data where I want to take the first observed ob score score for each individual id and subtract that from that last observed score.

The problem with asking for the first observation minus the last observation is that sometimes the first observation data is missing.

Is there anyway to ask for the first observed score for each individual, thus skipping any missing data?

I built the below df to illustrate my problem.

help <- data.frame(id = c(5,5,5,5,5,12,12,12,17,17,20,20,20),
                   ob = c(1,2,3,4,5,1,2,3,1,2,1,2,3),
                   score = c(NA, 2, 3, 4, 3, 7, 3, 4, 3, 4, NA, 1, 4))

   id ob score
1   5  1    NA
2   5  2     2
3   5  3     3
4   5  4     4
5   5  5     3
6  12  1     7
7  12  2     3
8  12  3     4
9  17  1     3
10 17  2     4
11 20  1    NA
12 20  2     1
13 20  3     4

And what I am hoping to run is code that will give me...

   id ob score  es
1   5  1    NA  -1
2   5  2     2  -1
3   5  3     3  -1
4   5  4     4  -1
5   5  5     3  -1
6  12  1     7   3
7  12  2     3   3
8  12  3     4   3
9  17  1     3  -1
10 17  2     4  -1
11 20  1    NA  -3
12 20  2     1  -3
13 20  3     4  -3

I am attempting to work out of dplyr and I understand the use of the 'group_by' command, however, not sure how to 'select' only first observed scores and then mutate to create es.

Answer 1

I would use first() and last() (both dplyr function) and na.omit() (from the default stats package.

First, I would make sure your score column was a numberic column with proper NA values (not strings as in your example)

help <- data.frame(id = c(5,5,5,5,5,12,12,12,17,17,20,20,20),
       ob = c(1,2,3,4,5,1,2,3,1,2,1,2,3),
       score = c(NA, 2, 3, 4, 3, 7, 3, 4, 3, 4, NA, 1, 4))

then you can do

library(dplyr)
help %>% group_by(id) %>% arrange(ob) %>% 
    mutate(es=first(na.omit(score)-last(na.omit(score))))

Answer 2

library(dplyr)

temp <- help %>% group_by(id) %>% 
     arrange(ob) %>%
     filter(!is.na(score)) %>% 
     mutate(es = first(score) - last(score)) %>%
     select(id, es) %>%
     distinct()

help %>% left_join(temp)