5. Accounting for Study Protocol

When you use an accelerometer in a study, you are likely to give your study participants specific instructions on when they should start wearing the accelerometer, for how many days, and whether or not they are expected to take the accelerometer off during specific activity types or parts of the day. Further, it may be that you turned on the accelerometers hours or even days before you gave it to the participant or stopped it hours or days after you received it back. Your knowledge about all these aspects of your study protocol can be used by GGIR to mask certain periods of time in the recording. This is important because this information is not necessarily obvious from the recorded data. For instance, when a recording is started and dispatched to the participant via mail, the time during which the devices are in transit and not worn may be impossible to distinguish from a participant wearing the accelerometer and commuting.

Selecting/Masking the data

It is important that GGIR masks all data outside the time window for which the participant was instructed to wear the accelerometer. Study protocols differ in duration and expected wear period, which is why GGIR offers a variety of ways to account for the study protocol.

The main parameter to do this is data_masking_strategy. It requires a numeric value indicating one of the following strategies:

Additionally, you can set the maximum duration the accelerometer is to be worn after recording starts. Use parameter maxdur to specify the duration in the number of 24 hour blocks or parameter max_calendar_daysfor the number of calendar days.

Key parameters