To evaluate the reliability of power systems, good estimates of the failure rates of the included components are needed. Better predictions can be performed if relevant factors that affect the failure rates are used, and an increasing number of models that include different types of factors have been presented recently. This paper proposes a categorization of failure rate factors into seven categories, based on the type of information for the factors. The categorization can be used to map future studies in the context of similar work.  Furthermore, the paper presents a review of a number of publications that uses different factors to model the failure rate of different power system components. The failure rate factors used in the reviewed publications are categorized into the proposed seven categories and a comprehensive summary table is included. The used models and methods to estimate the failure rate in the reviewed publications are also noted in the summary table.