This paper presents an analysis of the effects of shortcomings in the reporting of severe storm events on some common measures of warning performance. Such deficiencies lead to an apparent false alarm ratio (FAR) higher than the true value, and ordinarily to an apparent probability of detection (POD) also higher than the true value. An improved warning system may generate additional storm reports through closer collaboration between forecasters and storm spotters; an enhanced warning verification program will also tend to collect additional storm reports. Independently of any changes in the warning system, such additional reports tend to drive the apparent FAR down (and thus closer to the true value). If the verification efforts emphasize situations when warnings are in effect, the additional reports will further inflate the apparent POD. When changes occur in both the warning system and the verification program, the contributions of each to changes in the performance measures become intermingled. Understanding of these effects of the reporting system can aid in interpreting trends in the performance measures.

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