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Stratified random sampling from a `data.frame` in R
stratified <- function(df, group, size, select = NULL,
replace = FALSE, bothSets = FALSE) {
if (is.null(select)) {
df <- df
} else {
if (is.null(names(select))) stop("'select' must be a named list")
if (!all(names(select) %in% names(df)))
stop("Please verify your 'select' argument")
temp <- sapply(names(select),
function(x) df[[x]] %in% select[[x]])
df <- df[rowSums(temp) == length(select), ]
}
df.interaction <- interaction(df[group], drop = TRUE)
df.table <- table(df.interaction)
df.split <- split(df, df.interaction)
if (length(size) > 1) {
if (length(size) != length(df.split))
stop("Number of groups is ", length(df.split),
" but number of sizes supplied is ", length(size))
if (is.null(names(size))) {
n <- setNames(size, names(df.split))
message(sQuote("size"), " vector entered as:\n\nsize = structure(c(",
paste(n, collapse = ", "), "),\n.Names = c(",
paste(shQuote(names(n)), collapse = ", "), ")) \n\n")
} else {
ifelse(all(names(size) %in% names(df.split)),
n <- size[names(df.split)],
stop("Named vector supplied with names ",
paste(names(size), collapse = ", "),
"\n but the names for the group levels are ",
paste(names(df.split), collapse = ", ")))
}
} else if (size < 1) {
n <- round(df.table * size, digits = 0)
} else if (size >= 1) {
if (all(df.table >= size) || isTRUE(replace)) {
n <- setNames(rep(size, length.out = length(df.split)),
names(df.split))
} else {
message(
"Some groups\n---",
paste(names(df.table[df.table < size]), collapse = ", "),
"---\ncontain fewer observations",
" than desired number of samples.\n",
"All observations have been returned from those groups.")
n <- c(sapply(df.table[df.table >= size], function(x) x = size),
df.table[df.table < size])
}
}
temp <- lapply(
names(df.split),
function(x) df.split[[x]][sample(df.table[x],
n[x], replace = replace), ])
set1 <- do.call("rbind", temp)
if (isTRUE(bothSets)) {
set2 <- df[!rownames(df) %in% rownames(set1), ]
list(SET1 = set1, SET2 = set2)
} else {
set1
}
}
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