Safety instrumented systems often employ redundancy to enhance reliability, but the intended effect may be reduced when common cause failures are taken into account. It is often assumed that a certain fraction of component failures will occur close in time, due to a shared cause. Unfortunately, few attempts have been made to systematically investigate field experience on common cause failures, with the exception of the nuclear industry which has been in the forefront of research in this area. This paper presents selected results from a research project carried out in the Norwegian oil and gas industry to collect and analyze reported failures. This includes the presentation and derivation of generic (i.e. industry average) values of beta-factors for typical components in the oil and gas industry, and the demonstration of how failure data may be used to construct checklists for updating the value of beta in operation. The results are based on a review of some 12.000 maintenance notifications from six different onshore and offshore petroleum facilities. It is found that the new beta-values are higher than what is seen in many data sources, and some possible explanations are discussed.