public function getMeanGenerationTime(){
        if(!$this->is_setup) return;
        if(!$this->statistics_tab){
            $this->getStatisticsByGeneration();
        }
        //Linear regression on x=generation and y=birthdate
        $sum_xy = 0;
        $sum_x=0;
        $sum_y=0;
        $sum_x2=0;
        $n=count($this->statistics_tab);
        foreach($this->statistics_tab as $gen=>$stats){
            $sum_xy+=$gen*$stats['avgBirth'];
            $sum_x+=$gen;
            $sum_y+=$stats['avgBirth'];
            $sum_x2+=$gen*$gen;
        }
        $denom=($n*$sum_x2)-($sum_x*$sum_x);
        if($denom!=0){
            return -(($n*$sum_xy)-($sum_x*$sum_y))/($denom);
        }
        return null;
    }