ActiGraph ACTITRAINER Manuale Utente Pagina 22

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Determining Inventory Needs
A 7-day wear ties up 1 device for 11-14 days
1 day prep + 1 pre-wear day + 7 day wear + 1-4 post wear days +1 day download/charge
How many devices do we need for a 4 month data collection
period and 250 participants? Answer is 34
X = # days data collection/[days used per person] X = (120/12) = 10
Y = X * [loss rate] Y = (10*.03) =.30
Z = X-Y Z = (10 - .30) = 9.7
# participants to measure [final sample + rewears] 250 + (250*.3) = 325
Inventory = # of participants to measure/Z Inventory = (325/9.7) = 33.5
*Assumptions: 3% loss rate; 30% re-wear rate; average 12 day device time
Kelli L. Cain, San Diego May 2014
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