This paper offers a comprehensive study of survivorship issues, in the context of mutual fund research, using the mutual fund data set of Carhart (1997). We find that funds in our sample disappear primarily because of multi-year poor performance. Then we demonstrate analytically that this survival rule typically causes the survivor bias in average performance to increase in the length of the sample period, though it is possible to construct counterexamples. In the data, we find a strong positive relation between the survivor bias in average performance and sample period length. The bias is economically small at 17 basis points per annum for one-year samples, but a significantly larger one percent per annum for samples longer than fifteen years. We also find evidence of performance persistence in our sample and, consistent with the presence of a multi-period survival rule, we find that the persistence is weakened by survivorship bias. Finally, we explain how the relation between performance and fund characteristics can be affected by the use of a survivor-only sample and show that the magnitudes of the biases in the slope coefficients are large for fund size, expenses, turnover and load fees in our sample. Because survivorship issues are relevant for many data sets used in finance, the analysis in this paper has potential applications in areas of financial economics beyond just mutual fund research.1 Survivorship bias affects almost every mutual fund study. Most commercially available mutual fund data sets include only funds currently in operation, and many commonly used research methodologies impose additional selection biases. With the exception of a few recent papers, however, researchers frequently ignore selection biases altogether or argue that their effect is insignificant. This attitude is unfortunate, as selection-bias issues pervade almost all empirical studies of panel and time-series data sets. This paper offers a comprehensive study of survivorship issues, in the context of mutual fund research. We examine how survivorship bias affects mutual fund studies both theoretically and empirically. We study the effect of survivorship on three types of mutual fund studies: (1) estimates of average performance, (2) tests of performance persistence, and (3) cross-section estimates of the relation between performance and fund attributes. The analysis divides survivorship bias into the separate but related issues of survivor bias and look-ahead bias, an important distinction rarely acknowledged in the literature. Our results indicate that survivorship bias substantially alters the inferences from mutual fund studies, but that the effects vary across test type, form of survivorship bias, and sample period length. Because survivorship issues are relevant for many data sets used in finance, the analysis in this paper has potential applications in areas of financial economics beyond just mutual fund research.A number of recent papers have addressed issues in mutual fund survivorship. Brown, Goetzmann, Ibbotso...