New York City ? The housing slump isn’t over.

Tax credits and historically low mortgage rates have failed to lift home prices so far this year. Prices fell 0.5 percent in March from February, according to the Standard & Poor’s/Case-Shiller 20-city index released Tuesday.

That marks six straight months of declines — a sign that the housing market is going in reverse.

“It looks a little like a double-dip already,” economist Robert Shiller said in an interview. “There is a very real possibility of some more decline.”

The co-creator of the Case-Shiller index, who predicted in 2005 that the housing bubble would burst, says he worries that home prices rose last year only because of the federal tax credits. That fear is shared by other economists. They note that weak job growth, tight credit and millions more foreclosures ahead will weigh on the home market.