Comparing two visits in R

Question

I have a data with three variables. Patient ID as ID, Visit as Visit, and a third variable as Var. I have two visits and I would like to subset the IDs:

IDs that have missing value of Var in the Visit 2
IDs had a value 2 for Var in the Visit 1 but 1 in the Visit 2 and vice versa

ID Visit Var
1    1   2
1    2   2
2    1   1
2    2   2
3    1   2
3    2   1
4    1   2
4    2   NA

Does this answer your question? [Filter data.frame rows by a logical condition](https://stackoverflow.com/questions/1686569/filter-data-frame-rows-by-a-logical-condition) — s__, Oct 12 '21 at 12:26

score 0 · Accepted Answer · answered Oct 12 '21 at 12:30

You can subset using [ and using == for equal, & for and, is.na to test for missing values.

unique(x$ID[x$Visit == 2 & is.na(x$Var)])
#[1] 4

intersect(unique(x$ID[!is.na(x$Var) & x$Var == 2 & x$Visit==1])
        , unique(x$ID[!is.na(x$Var) & x$Var == 1 & x$Visit==2]))
#[1] 3

Data:

x <- read.table(header=TRUE, text="ID Visit Var
1    1   2
1    2   2
2    1   1
2    2   2
3    1   2
3    2   1
4    1   2
4    2   NA")

Comparing two visits in R

1 Answers1