A computer-implemented method for preventing inaccurate media measurement results, the method comprising: receiving media exposure data over a data network from at least one of a plurality of portable computing devices (601-604; 801-804); receiving motion strings over the data network respectively from each of the plurality of portable computing devices, the motion strings including a successive binary representation of motion over a first period of time; the method being characterized by further comprising: comparing, by executing an instruction with a processor, the motion strings received from two of the plurality of portable computing devices to determine if two of the motion strings match with each other within a predetermined threshold; identifying the two portable computing devices (601-604; 801-804) that produced matching motion strings as being physically carried by a same person, wherein: the first period of time includes a plurality of shorter periods of time, and the successive binary representation of motion includes a series of values representing motion for each of the shorter periods of time; each of the series of values are formed by determining if the gross motion within the shorter period of time exceeded a motion threshold; and a binary "1" is generated if the gross motion within the shorter time period meets or exceeds the motion threshold, and a binary "0" is generated if the gross motion within the shorter time period does not meet or exceed the motion threshold. receiving media exposure data over a data network from at least one of a plurality of portable computing devices (601-604; 801-804); receiving motion strings over the data network respectively from each of the plurality of portable computing devices, the motion strings including a successive binary representation of motion over a first period of time; the method being characterized by further comprising: comparing, by executing an instruction with a processor, the motion strings received from two of the plurality of portable computing devices to determine if two of the motion strings match with each other within a predetermined threshold; identifying the two portable computing devices (601-604; 801-804) that produced matching motion strings as being physically carried by a same person, wherein: the first period of time includes a plurality of shorter periods of time, and the successive binary representation of motion includes a series of values representing motion for each of the shorter periods of time; each of the series of values are formed by determining if the gross motion within the shorter period of time exceeded a motion threshold; and a binary "1" is generated if the gross motion within the shorter time period meets or exceeds the motion threshold, and a binary "0" is generated if the gross motion within the shorter time period does not meet or exceed the motion threshold. comparing, by executing an instruction with a processor, the motion strings received from two of the plurality of portable computing devices to determine if two of the motion strings match with each other within a predetermined threshold; identifying the two portable computing devices (601-604; 801-804) that produced matching motion strings as being physically carried by a same person, wherein: the first period of time includes a plurality of shorter periods of time, and the successive binary representation of motion includes a series of values representing motion for each of the shorter periods of time; each of the series of values are formed by determining if the gross motion within the shorter period of time exceeded a motion threshold; and a binary "1" is generated if the gross motion within the shorter time period meets or exceeds the motion threshold, and a binary "0" is generated if the gross motion within the shorter time period does not meet or exceed the motion threshold. the first period of time includes a plurality of shorter periods of time, and the successive binary representation of motion includes a series of values representing motion for each of the shorter periods of time; each of the series of values are formed by determining if the gross motion within the shorter period of time exceeded a motion threshold; and a binary "1" is generated if the gross motion within the shorter time period meets or exceeds the motion threshold, and a binary "0" is generated if the gross motion within the shorter time period does not meet or exceed the motion threshold.