private function getSumSquaredModel() : float
{
if (is_null($this->sumSquaredModel)) {
$average = array_sum($this->observations->getOutcomes()) / count($this->observations->getOutcomes());
$this->sumSquaredModel = static::sumSquaredDifference($this->getPredictedOutcomes(), $average);
}
return $this->sumSquaredModel;
}