double sum=0.0;
	for (std::vector<double>::const_iterator it=items_.begin(); it!=items_.end(); ++it)
		sum+=*it;
	m_=sum/double(items_.size());
	d_=0.0;	
	for (std::vector<double>::const_iterator it=items_.begin(); it!=items_.end(); ++it)
		d_+=pow(m_-(*it),2.0);
	d_/=double(items_.size());