WASHINGTON (AP) — Average long-term U.S. mortgage rates fell for the fourth straight week, with the benchmark 30-year rate again marking its lowest level since May 2013. The average for a 15-year mortgage, a popular choice for people who are refinancing, slipped further below 3 percent.

Mortgage company Freddie Mac said Thursday the nationwide average for a 30-year mortgage declined to 3.63 percent this week from 3.66 percent this week last week. The rate for the 15-year loan slipped to 2.93 percent from 2.98 percent last week.

A year ago, the average 30-year mortgage stood at 4.39 percent and the 15-year mortgage at 3.44 percent. Mortgage rates have remained low even though the Federal Reserve in October ended its monthly bond purchases, which were meant to hold down long-term rates.

The drop in mortgage rates has come as bond yields have hit record low levels. Mortgage rates often follow the yield on the 10-year Treasury note, which has fallen below 2 percent. Bond yields fall as prices rise.

The 10-year note traded at 1.87 percent Wednesday, up from 1.84 percent a week earlier but still at a historically low level. It dropped to 1.86 percent from 1.94 percent after Europe’s central bank announced a plan aimed at reviving that region’s struggling economy.

To calculate average mortgage rates, Freddie Mac surveys lenders across the country at the beginning of each week. The average doesn’t include extra fees, known as points, which most borrowers must pay to get the lowest rates. One point equals 1 percent of the loan amount.

The average fee for a 30-year mortgage was 0.7 point, up from 0.6 point last week. The fee for a 15-year mortgage rose to 0.6 point from 0.5 point.

The average rate on a five-year adjustable-rate mortgage fell to 2.83 percent from 2.90 percent. The fee remained at 0.4 point.

For a one-year ARM, the average rate was unchanged at 2.37 percent. The fee held at 0.4 point.