how to calculate the average difference between dates by ID in R

Question

I have the data set like below and i want to calculate the average time difference for each unique id

data:
   membership_id created_date 
1       12000000 2015         


        

Answer 1

Coming from plyr, you can probably transition very easily to dplyr. It won't be quite as fast as data table, but it will be much faster than ddply.

dat %>% group_by(membership_id) %>%
    arrange(created_date) %>%
    summarize(avg = as.numeric(mean(diff(created_date))))
# Source: local data frame [3 x 2]
#
#   membership_id   avg
#           (int) (dbl)
# 1      12000000   555
# 2      12000001   262
# 3      12000003   391

Without any more real effort, you can speed things up even more by converting to a data.table object but still use the dplyr commands. Pure data.table will still be even faster.

(Using this data)

dat = structure(list(membership_id = c(12000000L, 12000001L, 12000001L, 
12000001L, 12000001L, 12000003L, 12000003L, 12000000L, 12000000L
), created_date = structure(c(16455, 15663, 15985, 16135, 16449, 
15744, 16135, 16106, 15345), class = "Date")), .Names = c("membership_id", 
"created_date"), row.names = c("1", "2", "3", "4", "5", "6", 
"7", "8", "9"), class = "data.frame")