calculating count for a column of dates

Question

I want to calculate the mean and standard deviation for the number of dates (or visits) that people have. Sample data are:

id   date
1    2015-02-23
1    2015-04-24
2    2018-05-23
2    2022-12-05
2    2022-12-06
3    2021-05-21

ID1 has 2 visits (evidenced by 2 dates), ID2 has 3 visits, and ID3 has 1 visit, so the mean would be (2+3+1)/3 =2.

Does anyone know how to calculate the mean and standard deviation? I tried to do this using the summarize function, but I can't get it to work.

Is ID 1 has 3 visits or 2 ? – Mohamed Desouky Jun 28 '22 at 19:48 — Mohamed Desouky, Jun 28 '22 at 19:48
My mistake. I meant 2 visits. I updated my post. – D. Fowler Jun 28 '22 at 19:49 — D. Fowler, Jun 28 '22 at 19:49
An alternative might be `mean(table(df$id))` – markus Jun 28 '22 at 20:02 — markus, Jun 28 '22 at 20:02

score 0 · Accepted Answer · answered Jun 28 '22 at 19:53

You can use rle function with your id

data

df < - structure(list(id = c(1L, 1L, 2L, 2L, 2L, 3L), date = c("2015-02-23", 
"2015-04-24", "2018-05-23", "2022-12-05", "2022-12-06", "2021-05-21"
)), class = "data.frame", row.names = c(NA, -6L))

mean(rle(df$id)$lengths)

calculating count for a column of dates

1 Answers1