1. A method performed by a mobile device, the method comprising: receiving a plurality of sensor values detected by one or more sensors onboard the mobile device over a period of time; calculating a plurality of derived values from the plurality of sensor values; selectively combining the derived values to generate one or more abstract values; and identifying a presumed activity from a plurality of possible activities based on a level of similarity between the one or more abstract values and expected values of each of the plurality of possible activities that correspond to the one or more abstract values.