[SteveC]

Executive Summary: Historically, about two-thirds of all general aviation (GA) accidents that occur in instrument meteorological conditions (IMC) are fatal—a rate much higher than the overall fatality rate for GA accidents. The goal of this National Transportation Safety Board study was to better understand the risk factors associated with accidents that occur in weather conditions characterized by IMC or poor visibility (“weather-related accidents”). The study accomplished this goal using the case control methodology, which compared a group of accident flights to a matching group of nonaccident flights to identify patterns of variables that distinguished the two groups from each other. This methodology expands on previous Safety Board efforts that have typically concentrated on summaries of accident cases. The advantage of the case control methodology is that, instead of focusing on the factors that accidents have in common, and possibly being misled by characteristics common to most pilots/flights, it identifies characteristics that set accidents apart and contribute to their occurrence. For this study, Safety Board air safety investigators (ASI) collected data from 72 GA accidents that occurred between August 2003 and April 2004. When accidents occurred, study managers also contacted pilots of flights that were operating in the vicinity at the time of those accidents for information about their flight activity. A total of 135 nonaccident flights were included in the study.

(bold added for emphasis)

Here is the link to the summary page quoted above: "Risk Factors Associated with Weather-Related General Aviation Accidents"

There is a link on that page to the complete report. The report is a 1.9M pdf file, 80 pages long, but some interesting information.