protected function _stratify() : array
    {
        $strata = [];

        foreach ($this->labels as $index => $label) {
            $strata[$label][] = $this->samples[$index];
        }

        return $strata;
    }
protected function _stratify() : array
    {
        $strata = [];

        foreach ($this->labels as $index => $label) {
            $strata[$label][] = $this->samples[$index];
        }

        return $strata;
    }
protected function _stratify ( ) : array { $ strata = [ ] ; foreach ( $ this -> labels as $ index => $ label ) { $ strata [ $ label ] [ ] = $ this -> samples [ $ index ] ; } return $ strata ; }
Stratifying subroutine groups samples by label.

@throws \RuntimeException
@return array[]
Stratifying subroutine groups samples by label .