The number of single-family homes put under agreement in January was up 18.4 percent compared to the same time last year, the 21st straight month of year-to-year increases, the Massachusetts Association of Realtors said Tuesday.

The number of condos put under agreement in January was up 22.3 percent from the same month a year ago, the association said in a press release.

Kimberly Allard-Moccia. Photo taken from MAR’s website.

The association’s pending sales analysis counts homes that have been put under agreement but are not yet completed sales. Often six weeks can elapse between the time a home is put under agreement and the time that the sale closes. In general, economists look at closed home sales when they seek to evaluate the condition of the market, but those data points can be several weeks old. According to the association, pending sales can sometimes give more of a real-time indication of where the housing market might be heading.

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“The spring home buying season came early for many buyers in January as the numbers showed they made offers and took advantage of market conditions instead of waiting,” association president Kimberly Allard-Moccia said in a statement. “Late 2012 pending sales reports indicated that the closings for the first months of 2013 would most likely be up, but today’s report gives us reason to believe the same will be true about the early spring.”

The pending sales data are consistent with the most recent information about the Bay State’s housing market. Recent final-sales numbers issued by the Warren Group showed that the Massachusetts housing market made a comeback in 2012.