How did Nielsen get these numbers? According to their press release, "Nielsen GamePlay Metrics uses console data collected from the Nielsen's People Meter TV sample combined with Nielsen GamePlay Metrics' proprietary audio signature library that matches the unique audio signature of every game tracked on the six most widely available video game consoles, including PlayStation 2, PLAYSTATION 3, Xbox, Xbox 360, Wii and GameCube. The GamePlay Metrics user sample includes more than 12,000 households with approximately 33,000 individuals."
Although PS3 owners reported played the longest average sessions, they still account for only 1.5% of the total gameplay minutes played last month. The PS2 is still the most played console, accounting for more than 42% of total gameplay time.