The average interest rate for a 30-year mortgage dropped to a record low of 4.71 percent this week.This week's average rate of 4.71 percent, released Thursday by Freddie Mac, is the lowest since the mortgage finance company began tracking the data in 1971. The previous record of 4.78 percent was set during the week ending April 30 and matched last week.  For comparison, rates on 30-year mortgages in 2008 generally ranged from 6 percent to 7 percent. In 2003, widely recognized as a great year for mortgage rates, the range was 5 percent to 6 percent.