The nation’s average mortgage rates changed little from last week amid a mix of economic data reports, Freddie Mac said Thursday. The results of Freddie’s Primary Mortgage Market Survey revealed that the 30-year, fixed-rate mortgage averaged 3.99%, dropping below 4% for the second time this year. On Oct. 6 the rate was 3.94%, and last week it was 4%. This week’s 15-year FRM averaged 3.3%, down from last week when it averaged 3.31%. A year ago at this time, the 15-year FRM averaged 3.57%. Five-year, Treasury-indexed hybrid adjustable-rate mortgages averaged 2.98% this week, up from last week’s average of 2.96%. A year ago, the ARM averaged 3.25%. And one-year Treasury-indexed ARM averaged 2.95%, rising from last week when it averaged 2.88%. At this time last year, the 1-year ARM averaged 3.26%. Referencing data from the National Association of Realtors, Frank Nothaft, Freddie vice president and chief economist, said soft house prices and low mortgage rates have kept home-buyer affordability historically high. “In the third quarter, 74% of the NAR’s metropolitan areas exhibited annual house price declines compared to 72% in the second quarter,” Nothaft said. “In addition, 30-year, fixed mortgage rates averaged 4.3% in the third quarter as opposed to 4.7% in the second. These factors helped raise September’s NAR Housing Affordability Index to the third highest reading on record which dates back to 1971.” Results of Bankrate‘s weekly national survey of large lenders put the 30-year FRM at 4.25%, a 0.02% increase from last week. Fifteen-year FRM for the week is 3.5%, also up .02% from last week. And five-year ARM came in at 3.16%, down 0.03% from last week. Write to Justin T. Hilley. Follow him on Twitter @JustinHilley.