SIS Test Intervals – Know the Facts Before Changing , an article written by Kenexis President Ed Marszal, was published by ISS Source.  This article discusses how the failure rates that are utilized in SIL verification calculations are often based on the test intervals that are used at a facility and industry in general.  Simply using those failure rates with a modified test interval may result in erroneous results.  Caution should be used when extending test intervals.