The New York Times published an interesting article about module reliability and degredation problems, quoting several PVPMC members.  The article can be seen here:

One interesting and relevant fact cited is a study by Meteocontrol of 30,000 PV systems in Europe, which found that 80% of these systems are underperforming expectations.  This finding indicates to me that predicting module and systems degradation and defects is a critical need in the industry and care should be made to use the best data and models available when predicting PV performance.