Private Browsing was introduced in Firefox 3.5, giving users the option of browsing the web without keeping track of their history. A recent Test Pilot study recorded, among other things, the time users activated Private Browsing, and the time they deactivated it. Though what happens in Private Browsing stays in Private Private Browsing – that is, neither Firefox nor Test Pilot records anything during the period – we did learn a few things about the timing and duration of of Private Browsing mode sessions.

Here are a few simple insights we’ve gleaned from the data.

Note: Test Pilot is an opt-in service for Firefox 4 Beta users. We would never record a user’s activity for any reason unless they explicitly signed up for this study. Again, we do not record any information at all during a user’s Private Browsing session – only when private browsing was entered and exited. You can read over the Test Pilot Privacy Policy here.

Activation spikes at lunch.

Though people switch into Private Browsing mode throughout the day, there are a few periods where activation surges:

Lunch: users likely switch into Private Browsing during their lunch breaks. We see a major spike between 11am and 2pm. After School / Work: users appear to switch on Private Browsing just after they’ve returned from work or school, which is around 5pm. After Dinner: there is another substantial usage peak between nine and ten pm. Late Night: a minor spike exists an hour or two after midnight.

The 10-minute window is the most common.

Now that we know when users jump into Private Browsing, the next question involves finding out how long users stay in it. The 25th percentile stays on for about 4 and a half minutes, the 75th percentile around 22 minutes, and the median stays in for about 10 minutes.

This trend appears to hold over the entire course of the day, with the notable exception of 5pm. For some reason the median and lower quantiles are lower than the other hours.

We’ve more insights to glean from the Week-In-The-Life study. Stay tuned for more.