I like having statistics and metrics to refer to about all kinds of things. I guess that's why I spend a lot of time taking a bunch of statistics courses in graduate school. So, I'm always happy to see items like this one from The Wall Street Journal...

Report: iPhone Users Spend 9.6 Minutes per App

...even though I know I'll probably end up offering caveats for someone else's interpretation of such data. It references this report from Greystripe titled...

Greystripe Consumer Insights Report (April 2009) (PDF)

It is a beautifully illustrated report with easy to read and understand charts and tables.

Caption from page 13 of Greystripe's report

As you can see from the sample caption in Greystripe's report they provide sample size information (N-210 for this particular item) and are always careful to say that each statistic/metric is from/about Greystripe's users. They make no claims about generalizing their findings to all iPhone apps. In fact, the number the Wall Street Journal highlighted (9.6 minutes per iPhone app use) is specifically about Greystripe's free ad-supported games.

The moral to this story is always be careful about various interpretations of mobile app metrics. And, do take a look at Greystripe's report. It really is very interesting and informative.