If you use your iPhone to monitor your health, it may be giving you incorrect data.

A  new study found that while the built in pedometer is a great idea, it’s underestimating how many steps you take. The study found the app is accurate in the lab, but in the real world it misses about 21.5 per cent of a user’s steps. Lead author  Mark Duncan of the University of British Columbia says although the iPhone pedometer isn’t perfect,  it’s still valuable for monitoring and encouraging activity.