Hi John,
When doing analysis on days defined from midnight-to-midnight (MM) in GGIR part 5 days will by default only be included if they have at least 23 hours of data (any data, valid or not valid) which can be modified with argument minimum_MM_length.part5. This applies as of GGIR 2.0.
However, I personally think it is not a good idea to include incomplete days in part 5 MM analysis. Time-use in an incomplete day is not comparable to time-use in a complete data. So, person-level averages will be biased if you combine them. In other words it is not fair to compare the fact that I was inactive in half day recording with a day on which I had full data. The magnitude of acceleration imputation we use in GGIR data does not address this issue, because imputation is only useful for measures like average acceleration, not for time-use variables. We would need an imputation of the behavioral sequence to include those incomplete days. To my knowledge this does not exist yet. Anyone interested in working on this, please reach out to me. Further, if the recording starts and stops at a midnight the sleep estimates may not be reliable for the first half and last half night. Therefore, my advice would be: If you want to work with 7 MM days in part 5, collect at least an extra 8 hours of data before and after it.
Part 2, 4 and 5 will result in a different number of days/nights because each part has a different purpose. In part 2 we are not so much interested in time-use and more on average and distribution of acceleration, which are more compatible with the imputation we use. In part 4 we only care about whether sleep can be detected. In part 5, both sleep and daytime time use estimates need to be complete for the majority of the time.
If you have suggestions on how we can better utilize those incomplete days of data then let me know. Otherwise I think the best advise is to aim for longer recordings than just the commonly used 7 days.
Vincent
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