Interpreting Vigorous Min vs ACC output

[Reagan Moffit]

Hello, 

I have a quick question on interpreting output. For my study, I have 746/818 participants with missing values for the variable "mean acceleration in time spent vigorous." I had originally thought that the values were missing because the participant did not accumulate any time spent in vigorous activity (our sample is older adults and we used the default threshold for vigorous [400] so this makes sense). However, of the 746 with missing ACC (mg) in Vig time, 115 has >0 minutes of time (min) spent in vigorous activity. Does anyone know why that is? 

My two thoughts: 

1) Vig time (min) is still low, so not enough time spent in vigorous time

2) Maybe this is related to the EPOCH size and time criteria parameter? We used 60 sec epochs and used the defaults for mvpa.criter. 

GGIR version 3.1.1 and R version 4.3.1.

Thank you!

Reagan

[Vincent van Hees]

Hi Reagan,

Assuming this question relates to part5 csv output:

dur_... variables such as dur_spt_wake_VIG_min and dur_day_total_VIG_min always have a value because even 0 minutes spent in a behaviour is informative.

ACC_... values are empty when no time was spent in a behavioural category, because average acceleration could not be calculated as there was no time spent in that behaviour.

I did this on purpose because filling in zero would be incorrect and skew the person level aggregates. For example, if a person typically moves with an acceleration of 300 during vigorous activity, but does not do vigorous activity on one day of the week, then the weekly average would indicate that they moved with 257 acceleration. That is inaccurate, and does not reflect the person's try typical acceleration in vigorous activity. On the other hand their typical time spent in vigorous activity should take into account the zero minutes spent on one of the days.

Maybe a metaphor helps: SpaceX shoots their rockets into space with a 4g acceleration but on many days of the year they do not shoot rockets into space, it would be meaningless to then say that the typical acceleration of SpaceX rocket launches is 0.1g only because all those days without a launch are allowed to be represented by 0 acceleration.

The advantage of this approach is further that by multiplying acceleration and duration you can get a volume measure of behaviour in that specific category. If we interpret acceleration as a proxy for energy expenditure per minute then this product would be a proxy for total calories burned. The unit of measurement would not be calories but the construct would be similar.

Best, Vincent

Dr. Vincent van Hees | Independent consultant | https://accelting.com/

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