Take randomly sample based on groups

Question

I have a df made by almost 50,000 rows spread in 15 different IDs (every ID has thousands of observations). df looks like:

        ID  Year    Temp    ph
1           


        

Answer 1

In case you have big datasets, a data.table solution could go like this:

library(data.table)

# Generate 26 mil rows random data
set.seed(2019)
dt <- data.table(c1 = sample(length(LETTERS)*10^6), 
                 c2 = sample(LETTERS, replace = TRUE))

# For each letter, sample 500 rows
dt_sample <- dt[, .SD[sample(x = .N, size = 500)], by = c2]

# We indeed sampled 500 rows for each letter
dt_sample[, .N, by = c2][order(c2)]
#>     c2   N
#>  1:  A 500
#>  2:  D 500
#>  3:  G 500
#>  4:  I 500
#>  5:  M 500
#>  6:  N 500
#>  7:  O 500
#>  8:  P 500
#>  9:  Q 500
#> 10:  R 500
#> 11:  S 500
#> 12:  T 500
#> 13:  U 500
#> 14:  V 500
#> 15:  W 500
#> 16:  Y 500
#> 17:  Z 500

^{Created on 2019-04-23 by the reprex package (v0.2.1)}

In case your data is unbalanced in the sense that some groups happen to be smaller (as number of rows) than your desired sample size, then you need to set a defensive trick like sample size should be min(500, .N) - see sample random rows within each group in a data.table. So like:

dt[, .SD[sample(x = .N, size = min(500, .N))], by = c2]