public function portfolioStdDeviation()
{
$portfolio = new table_Portfolio();
$w = $portfolio->weights();
$n = 0;
foreach ($w as $i) {
$weights[$n] = $i;
$n++;
}
$rets = $this->ReturnsArray();
$j = 0;
foreach ($rets as $r) {
$returns[$j] = $r;
$j++;
}
$matrix = new Finance_Matrix($weights, $rets);
$variance = $matrix->variancePortfolio($weights, $returns);
$portfolioStdDeviation = sqrt($variance);
return $portfolioStdDeviation;
}
