With regard to the previous discussion I'm using the first solution from AlexisW to solve this problem in my full data set. Although after I pivot my data frame and try to create a column of indexes and use that to select every fifth measurement, I get the error seen in the screenshots. Can anyone suggest a potential solution? I don't know why it is off 4 values (must be size 3264 or 1, not 3260)

It appears the number of rows in the tibble is not a multiple of 5, hence the error. Another option would be to use modular arithmetic via row numbers to repeat 1-5 in a new index column. Below is an example of how to do that (note the number of rows is not a multiple of 5).

library(tidyverse)
df = data.frame(id = letters[1:12])
out = df |>
mutate(index = ifelse(row_number() %% 5 == 0,
5,
row_number() %% 5))
out
#> id index
#> 1 a 1
#> 2 b 2
#> 3 c 3
#> 4 d 4
#> 5 e 5
#> 6 f 1
#> 7 g 2
#> 8 h 3
#> 9 i 4
#> 10 j 5
#> 11 k 1
#> 12 l 2

If you just want to select every fifth row, then you can do the following.

df |>
filter(row_number() %% 5 == 0)
#> id
#> 1 e
#> 2 j