By Prince McLean

Published: 12:00 PM EST (09:00 AM PST)

An analysis of NPD market research data from all three months of Apple's December quarter suggests sales of around 16.5 million iPods, according to research and investment firm PiperJaffray.

Analyst Gene Munster, in his second of two research notes on the Cupertino-based company Wednesday, told clients that previous assumptions of 15.5 million to 16.0 million were based off only the first two months (Oct., Nov.) of NPD data.

"The [approximate] 16.5 million expectation is based on various assumptions, so we believe investors should supplement this data point with other information," he wrote.

Munster said it's also important that investors expect an approximate 10 percent decline in the average-selling-price of iPods during the December quarter, primarily due to strong sales of the new low-cost ($79) iPod shuffle.

Apple will release results for the quarter -- its first of fiscal 2007 -- after the close of the stock market today.

In an earlier note, Munster said NPD data coupled with his checks at 20 Apple specialist stores leads him to believe Apple will report slight upside across the board. He expects the company to beat estimates for both Mac and iPod unit estimates and overall consensus earnings-per-share and revenue