Madoff's annual returns were "unusually consistent",around 10%, and were a key factor in perpetuating the fraud. Ponzi schemes typically pay returns of 20% or higher, and collapse quickly. One Madoff fund, which described its "strategy" as focusing on shares in the Standard & Poor's 100-stock index, reported a 10.5% annual return during the previous 17 years. Even at the end of November 2008, amid a general market collapse, the same fund reported that it was up 5.6%, while the same year-to-date total return on the S&P 500-stock index had been negative 38%.An unnamed investor remarked, "The returns were just amazing and we trusted this guy for decades if you wanted to take money out, you always got your check in a few days. That's why we were all so stunned."

The Swiss bank Union Bancaire Prive explained that because of Madoff's huge volume as a broker-dealer, the bank believed he had a perceived edge on the market because his trades were timed well, suggesting they believed he was front running.

what was the monthly average return and how long did the monthly average return last?