The EPA recently finalized changes to the data handling conventions and computations necessary for determining when the annual and 24-hour national ambient air quality standards (NAAQS) for fine particles (generally referring to particles less than or equal to 2.5 micrometers ([mu]m) in diameter, PM<INF>2.5</INF>) are met. These changes were made in support of revisions to the NAAQS for particulate matter (PM) that were finalized in the same rulemaking. After publication, EPA discovered an inadvertent omission in the rule text explaining the procedures for calculating the key statistic (98th percentile) involved with determining compliance with the 24-hour PM<INF>2.5</INF> standard in locations where extra samples of PM<INF>2.5</INF> in ambient air were taken above the specified sampling frequency. If the error in the regulatory text is left unchanged, the resulting statistic for calculating compliance with the 24-hour PM<INF>2.5</INF> standard would be biased low at some samplers, leading to potentially incorrect determinations that an area was attaining the NAAQS. In this direct final action, EPA is correcting this error. The correction involves the replacement of the currently used statistical formula and instructions with a simpler look-up table approach which is easier for readers to understand and which retains the intended numerical consistency with EPA's historic practice.