Questions about data format

Discusss here aspects related to the challenge dataset format, or link to tools, scripts, to assist in the data access.
 
DATA FORMAT
Q: I noticed that across the different datasets from all the subjects (both ADL and DRILL) there are plenty of portions where huge chunks of sensor data (mainly the accelerometers on the upper limbs) are missing, i.e. those columns contain NaNs. What happened there?
A: NaN values correspond to missing samples in the wireless sensors. These are Bluetooth accelerometers that suffered a few disconnections with loss of data chunks (we had too many wireless things around).
 
LABELS

Q: Would you please explain the meaning of a zero value in either of the last two columns?  Would it mean that there is no annotation available for the sensor instance?
A: Exactly, a zero in the last columns means that no label is available for that particular sample (each row is a sample, with 30Hz rate). You can consider it like a "null class".
Concerning the posture/locomotion there are zeros mainly in transition phases, for example when the subject was first standing and then sitting down, you will see a label for standing, then a few zeros and finally a label for sitting. This is to avoid introducing samples which would not be representative of the activity. Regarding the gesture labels, there you have zeros whenever the corresponding gesture is not among the ones listed in the challenge description.