Random sampling with optional stratification

Usage

sample_data(
  data,
  group = NULL,
  size = NULL,
  frac = NULL,
  replace = FALSE,
  seed = NULL,
  verbose = FALSE
)

Arguments

data: A data frame.
group: Grouping column.
size: Sample size per group (see Details).
frac: Sampling fraction per group (see Details).
replace: Sample with replacement.
seed: Random seed. If NULL, the current RNG state is used; if supplied, set.seed(seed) is called before sampling.
verbose: Logical.

Value

A sampled data frame.

Details

If group is NULL, either size (absolute number of rows) or frac (fraction of nrow(data)) must be supplied.

If group is provided, sampling is performed separately within each group. With size, every group contributes the same absolute number of rows (capped at the group's own size unless replace = TRUE). With frac, every group contributes its own round(frac * group_size) rows (at least 1), so groups of different sizes are sampled proportionally. If group is provided but both size and frac are NULL, each group contributes exactly one row.

Examples

sample_data(mtcars, frac = 0.5)
#>     mpg cyl  disp  hp drat    wt  qsec vs am gear carb
#> 1  10.4   8 472.0 205 2.93 5.250 17.98  0  0    3    4
#> 2  33.9   4  71.1  65 4.22 1.835 19.90  1  1    4    1
#> 3  26.0   4 120.3  91 4.43 2.140 16.70  0  1    5    2
#> 4  19.7   6 145.0 175 3.62 2.770 15.50  0  1    5    6
#> 5  15.2   8 304.0 150 3.15 3.435 17.30  0  0    3    2
#> 6  14.3   8 360.0 245 3.21 3.570 15.84  0  0    3    4
#> 7  18.1   6 225.0 105 2.76 3.460 20.22  1  0    3    1
#> 8  10.4   8 460.0 215 3.00 5.424 17.82  0  0    3    4
#> 9  24.4   4 146.7  62 3.69 3.190 20.00  1  0    4    2
#> 10 19.2   6 167.6 123 3.92 3.440 18.30  1  0    4    4
#> 11 30.4   4  75.7  52 4.93 1.615 18.52  1  1    4    2
#> 12 22.8   4 108.0  93 3.85 2.320 18.61  1  1    4    1
#> 13 17.8   6 167.6 123 3.92 3.440 18.90  1  0    4    4
#> 14 15.8   8 351.0 264 4.22 3.170 14.50  0  1    5    4
#> 15 15.0   8 301.0 335 3.54 3.570 14.60  0  1    5    8
#> 16 21.5   4 120.1  97 3.70 2.465 20.01  1  0    3    1
sample_data(mtcars, size = 3, group = "cyl", seed = 123)
#>    mpg cyl  disp  hp drat    wt  qsec vs am gear carb
#> 1 19.7   6 145.0 175 3.62 2.770 15.50  0  1    5    6
#> 2 21.4   6 258.0 110 3.08 3.215 19.44  1  0    3    1
#> 3 17.8   6 167.6 123 3.92 3.440 18.90  1  0    4    4
#> 4 30.4   4  95.1 113 3.77 1.513 16.90  1  1    5    2
#> 5 24.4   4 146.7  62 3.69 3.190 20.00  1  0    4    2
#> 6 33.9   4  71.1  65 4.22 1.835 19.90  1  1    4    1
#> 7 13.3   8 350.0 245 3.73 3.840 15.41  0  0    3    4
#> 8 15.2   8 275.8 180 3.07 3.780 18.00  0  0    3    3
#> 9 17.3   8 275.8 180 3.07 3.730 17.60  0  0    3    3

# frac is per-group: large groups contribute more rows
n_by_cyl <- function(d) table(d$cyl)
n_by_cyl(sample_data(mtcars, frac = 0.5, group = "cyl", seed = 1))
#> 
#> 4 6 8 
#> 6 4 7