Methods and apparatus to detect carrying of a portable audience measurement device

1. A portable audience measurement device, comprising:

a housing;

a media detector in the housing to collect media exposure data;

an attachment mechanism coupled to the housing to secure the housing to a person, wherein the attachment mechanism comprises a hinge in and a sensor coupled to the hinge pin;

the sensor comprising a rotary indicator to detect an angular position of an arm of the attachment mechanism relative to a surface of the housing to indicate a magnitude of rotation of the hinge pin; and

a detector to determine whether the device is being carried by the person based on the angular position of the arm by comparing the angular position of the arm to a threshold angle corresponding to the device.

2. A portable device as defined in claim 1 , wherein the attachment mechanism comprises a spring, and further comprising a second sensor coupled to the spring.

3. A portable device as defined in claim 2 , wherein the second sensor is to detect a tension of the spring indicative of whether the attachment mechanism is attached to an object.

4. A portable device as defined in claim 2 , wherein the second sensor comprises a strain gauge.

5. A portable device as defined in claim 2 , wherein the spring is to bias the arm of the attachment mechanism away from the housing, and further comprising a clasp to secure the attachment mechanism in a closed position.

6. A portable device as defined in claim 1 , wherein the arm of the attachment mechanism is to contact the housing in a closed position, and further comprising a first contact sensor disposed on a face of an end of the arm and a second contact sensor disposed on the housing in juxtaposition with the first contact sensor such that the first and second contact sensors cooperate to detect closure of the attachment mechanism.

7. A portable device as defined in claim 1 , wherein the attachment mechanism comprises a spring and the arm is to contact the housing in a closed position, and further comprising a first contact sensor disposed on a face of an end of the arm and a second contact sensor disposed on the housing in juxtaposition with the first contact sensor such that the first and second contact sensors cooperate to detect closure of the attachment mechanism, and a force sensor coupled to the spring to detect a tension of the spring indicative of whether the attachment mechanism is attached to an object.

8. A portable device as defined in claim 1 , wherein the rotary indicator comprises a light source and a photodetector, wherein the photodetector is to sense light conveyed from the light source through a pattern disc onto the photodetector.

9. A portable device as defined in claim 1 , wherein the detector is to calculate a likelihood that the device was being worn by the person during a period of time based on an output of the sensor.

10. A portable device as defined in claim 9 , wherein the detector is to credit the media exposure data collected during the period of time when the likelihood is greater than a threshold.

11. A portable device as defined in claim 9 , wherein the detector is to not credit the media exposure data when the likelihood is less than a threshold.

12. A portable device as defined in claim 9 , wherein the likelihood comprises a primary likelihood based on information gathered by the sensor, wherein the detector is to adjust the primary likelihood based on an output of a secondary sensor indicative of a position of the attachment mechanism.

13. A method of detecting wearing of a portable audience measurement device, comprising:

calculating, with a logic circuit, a magnitude of rotation of a hinge pin of an attachment mechanism of the portable audience measurement device based on an output from a sensor associated with the hinge pin, wherein the sensor comprises a rotary indicator to indicate the magnitude of rotation of the hinge pin;

determining, with the logic circuit, whether an arm of the attachment mechanism is in an angular position indicative of the device being worn by a person based on the magnitude of rotation by comparing the angular position of the arm to a threshold angle corresponding to the portable audience measurement device;

calculating, with the logic circuit, a frequency of indications that the device is being worn by the person for a period of time; and

calculating, with the logic circuit, a likelihood that the device is being worn by the person based on the frequency.

14. A method as defined in claim 13 , further comprising comparing the likelihood to a threshold.

15. A method as defined in claim 14 , further comprising generating a message of non-compliance in response to determining that the likelihood is lower than the threshold.

16. A method as defined in claim 14 , further comprising comparing a time of the last detected actuation of the attachment mechanism to a threshold in response to determining that the attachment mechanism is in a position indicating that the device is not being worn by the person.

17. A method as defined in claim 16 , further comprising generating a message of non-compliance in response to determining that the time is greater than the threshold.

18. A method as defined in claim 13 , further comprising using the likelihood to determine whether collected media measurement information is to be credited.

19. A method as defined in claim 13 , wherein a change in a status of the attachment mechanism as detected by a sensor indicates actuation of the attachment mechanism by the person.

20. A method as defined in claim 13 , further comprising conveying the calculated likelihood to a central facility.

21. A method of crediting audience measurement data, comprising:

collecting audience measurement data with a portable device;

sensing a first angular position of an arm of an attachment mechanism of the portable device using a primary sensor, the attachment mechanism comprising a hinge pin monitored by the primary sensor, the primary sensor comprising a rotary indicator to indicate a magnitude of rotation of the hinge in corresponding to the first angular position;

calculating a likelihood that the attachment mechanism is attached to an object based on information related to the first angular position of the attachment mechanism by comparing the first angular position of the arm to a threshold angle corresponding to the device;

sensing a second position of the attachment mechanism using a secondary sensor;

adjusting the likelihood that the attachment mechanism is attached to the object based on the second position of the attachment mechanism; and

invalidating the audience measurement data for a time period in which the likelihood for the time period is below a threshold.

22. A method as defined in claim 21 , wherein the secondary sensor comprises a first contact sensor coupled to a body of the device and a second contact sensor coupled to the arm of the attachment mechanism.

23. A method as defined in claim 22 , wherein the first and second contact sensors are to detect whether the arm of the attachment mechanism is in contact with the body of the device.