名詞「incident」と「incidence」はつづりも発音もよく似ていますが、意味は大きく異なります*。以下に2語の定義と正しい用法を例示します。
incident:（好ましくない）出来事。
(1) In that incident, the level of contamination was between 6 and 16 ppm.
(2) Most such incidents result in miscarriage.
incidence:（病気などの）発生、発生率。
(3) The incidence of both of these diseases increased in the U.S. between 1994 and 2007.
(4) The city has implemented measures to reduce the incidence of accidents at this intersection.
*2語ともここで例示する以外にもいくつか意味がありますので、ご注意ください。