Effectiveness of Home Mortgage Modifications

Is mortgage modification effective at preventing foreclosure? I have been looking for statistics regarding the effectiveness of mortgage modifications and have not found any good evidence of the effectiveness of the program. I can report that with my clients most receive a small reduction in their mortgage payments, but most do not receive any large payment reductions with mortgage modifications. The process also seems to take between six months and a year to complete. Lenders forgiving principal on the loans is not common, which is probably the most effective way of preventing foreclosure.