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Although sustained superior firm performance may arise from skillful management or other valuable, rare, and inimitable resources, it can also result from randomness. Studying U.S. companies from 1965-2008, we benchmark how long a firm must perform at a high level to be confident that it is something other than the outcome of a time-homogeneous stationary Markov chain defined on the state space of percentiles. We find (a) the number of sustained superior performers in Compustat, measured by ROA and Tobin's q, exceeds the number of false positives we would expect to be generated by such a process; yet (b) the occurrence of false positives is often enough to fool many observers, so (c) the identification of sustained superior performers requires particularly stringent benchmarks to enable valid study.

Objective -To make recommendations for the National Institute for Health and Care Excellence (NICE) on the factors to consider when choosing databases and search techniques when producing systematic reviews to support public health guidance development.Methods -Retrospective analysis of how the publications included in systematic reviews commissioned by NICE on obesity, spatial planning, and tuberculosis were retrieved. The Evidence Based Library and Information Practice 2015, 10.1 51 included publications were checked to see if they were found from searching MEDLINE, another database or through other search techniques.Results -MEDLINE contributed 24.2% of the publications included in the obesity review, none of the publications in the spatial planning review and 72% of those in the tuberculosis review. Other databases accounted for 9.1% of included publications in obesity, 20% in spatial planning and 4% in tuberculosis. Non-database methods provided 42.4% of the included publications in the obesity review, compared to 5% in the spatial planning review and 24% in the tuberculosis review. It was not possible to establish retrospectively how 24.2% of the publications in the obesity review and 75% in the spatial planning review were found.Conclusions -Topic-specific databases and non-database search techniques were useful for tailoring the resources to the review questions. The value of MEDLINE in these reviews was affected by the degree of overlap with clinical topics, the domain of public health, and the need to find grey literature. The factors that NICE considers when planning a systematic search are the multidisciplinary nature of public health and the different types of evidence required.