Get the aggregation value for BMI with respect to the previous
        time interval.

def aggregations(self):
        """Get the aggregation value for BMI with respect to the previous
        time interval."""

        prev_month_start = get_prev_month(self.end,
                                          self.closed.query.interval_)
        self.closed.query.since(prev_month_start,
                                field="closed_at")
        closed_agg = self.closed.aggregations()
        self.opened.query.since(prev_month_start)
        opened_agg = self.opened.aggregations()
        if opened_agg == 0:
            bmi = 1.0  # if no submitted issues/prs, bmi is at 100%
        else:
            bmi = closed_agg / opened_agg
        return bmi