A quintile is simply one-fifth of a ranked list. Quintiles are determined by ranking counties in order by rate. The rates are sorted from the highest (1) to the lowest (115) rate.
Since Missouri has 115 counties (including the City of St. Louis, which is an independent city), there are 23 counties in each quintile.
In cases where the numbers of events are small, a county may fall into the highest or lowest quintile on the basis of a few events, even if the underlying true rate is not particularly high or low. Therefore, quintiles are not shown if fewer than half of the counties have 20 or more events. A value of 'N' or a blank is displayed to indicate this.