/**
* Stratified sampling for classifiers
*
* @param fr Input frame
* @param label Label vector (must be enum)
* @param sampling_ratios Optional: array containing the requested sampling ratios per class (in
* order of domains), will be overwritten if it contains all 0s
* @param maxrows Maximum number of rows in the returned frame
* @param seed RNG seed for sampling
* @param allowOversampling Allow oversampling of minority classes
* @param verbose Whether to print verbose info
* @return Sampled frame, with approximately the same number of samples from each class (or given
* by the requested sampling ratios)
*/
public static Frame sampleFrameStratified(
final Frame fr,
Vec label,
float[] sampling_ratios,
long maxrows,
final long seed,
final boolean allowOversampling,
final boolean verbose) {
if (fr == null) return null;
assert (label.isEnum());
assert (maxrows >= label.domain().length);
long[] dist = new ClassDist(label).doAll(label).dist();
assert (dist.length > 0);
Log.info(
"Doing stratified sampling for data set containing "
+ fr.numRows()
+ " rows from "
+ dist.length
+ " classes. Oversampling: "
+ (allowOversampling ? "on" : "off"));
if (verbose) {
for (int i = 0; i < dist.length; ++i) {
Log.info(
"Class "
+ label.domain(i)
+ ": count: "
+ dist[i]
+ " prior: "
+ (float) dist[i] / fr.numRows());
}
}
// create sampling_ratios for class balance with max. maxrows rows (fill existing array if not
// null)
if (sampling_ratios == null
|| (Utils.minValue(sampling_ratios) == 0 && Utils.maxValue(sampling_ratios) == 0)) {
// compute sampling ratios to achieve class balance
if (sampling_ratios == null) {
sampling_ratios = new float[dist.length];
}
assert (sampling_ratios.length == dist.length);
for (int i = 0; i < dist.length; ++i) {
sampling_ratios[i] =
((float) fr.numRows() / label.domain().length) / dist[i]; // prior^-1 / num_classes
}
final float inv_scale =
Utils.minValue(
sampling_ratios); // majority class has lowest required oversampling factor to achieve
// balance
if (!Float.isNaN(inv_scale) && !Float.isInfinite(inv_scale))
Utils.div(
sampling_ratios,
inv_scale); // want sampling_ratio 1.0 for majority class (no downsampling)
}
if (!allowOversampling) {
for (int i = 0; i < sampling_ratios.length; ++i) {
sampling_ratios[i] = Math.min(1.0f, sampling_ratios[i]);
}
}
// given these sampling ratios, and the original class distribution, this is the expected number
// of resulting rows
float numrows = 0;
for (int i = 0; i < sampling_ratios.length; ++i) {
numrows += sampling_ratios[i] * dist[i];
}
final long actualnumrows = Math.min(maxrows, Math.round(numrows)); // cap #rows at maxrows
assert (actualnumrows
>= 0); // can have no matching rows in case of sparse data where we had to fill in a
// makeZero() vector
Log.info("Stratified sampling to a total of " + String.format("%,d", actualnumrows) + " rows.");
if (actualnumrows != numrows) {
Utils.mult(
sampling_ratios,
(float) actualnumrows
/ numrows); // adjust the sampling_ratios by the global rescaling factor
if (verbose)
Log.info(
"Downsampling majority class by "
+ (float) actualnumrows / numrows
+ " to limit number of rows to "
+ String.format("%,d", maxrows));
}
Log.info(
"Majority class ("
+ label.domain()[Utils.minIndex(sampling_ratios)].toString()
+ ") sampling ratio: "
+ Utils.minValue(sampling_ratios));
Log.info(
"Minority class ("
+ label.domain()[Utils.maxIndex(sampling_ratios)].toString()
+ ") sampling ratio: "
+ Utils.maxValue(sampling_ratios));
return sampleFrameStratified(fr, label, sampling_ratios, seed, verbose);
}
