Like the trail of crumbs in the tale of Hansel and Gretel, data trickling from the gadgets we carry in our handbags and pockets map our every move. By adding GPS readings to output from the motion sensors that are becoming increasingly common in cellphones, it is possible to build up a meticulous record of an individual's day-to-day activity.rnThis can now be exploited by apps on the phone to predict what services we will require or to spot abnormal activity that could indicate we need help (see page 24). Yet just as Hansel's breadcrumb trail failed to save the pair when birds ate it, its modern equivalent could go equally badly wrong.
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