[Gamasutra analyst Matt Matthews examines hardware, software and accessory trends for U.S. video game retail sales in May -- a month what NPD Group called "the lowest month of sales for the industry since October 2006."]

After a stunning April the retail video game industry saw revenue decline 13 percent in May, with revenue declining across all segments according to the latest estimates released by the NPD Group on Monday. Only Nintendo's Wii saw its hardware sales increase month-over-month, and that system had the benefit of a price drop.

Even the accessory segment, which had seen year-to-date revenue up by 15 percent buoyed by Xbox Kinect sales, suffered a six percent drop in revenue compared to May 2010.

Along with a quick look at the top-line sales figures, we will dig into the many changes in the industry over the past month, including the effect that the PlayStation Network outage may have had on Sony's retail sales, the historical context for May's revenue figures, and the prospects for the Nintendo Wii and 3DS in the coming months.

Industry At a Glance

One could be forgiven for thinking the physical retail industry had turned a corner a month ago when we were considering April's sales estimates. In terms of dollars, total sales were up nearly 22 percent and software was up over 26 percent.

Compare that, now, with the dismal 13 percent decline overall and 19 percent decline in software for the month of May.

Despite the fall in monthly revenue, the year-to-date figures give room for cheer. Total industry revenue is actually flat, compared with the same period last year, and software is down a much more modest 6 percent in terms of dollars. In terms of units, software is down only 3 percent, showing that software prices have generally declined in 2011.

The table below summarizes the key figures.

For the record, the NPD Group is now clearly labeling their media releases to indicate that the figures include only retail sales. They are providing quarterly estimates of extra-retail sales, including mobile games, downloadable content, and casual games, along with other segments, but those figures are not included in the figures above nor are they considered directly in this analysis.