The estimation technique begins with the assumption that the total number of lost hours due to illness and disability for a given survey reference week is representative of the lost hours in the rest of that month. [...] Our estimation technique begins with the assumption that the number of overtime hours worked in a given survey reference week is representative of the overtime hours worked in the rest of that month. [...] Aggregate overtime hours for each month of each survey year were calculated using the formula: (aggregate hours of overtime in the survey reference week ÷ 7 days) × the number of days in the survey month. [...] The rates of unpaid overtime and paid overtime both increased since 1997, from 7.1% to 12.8% in the case of unpaid overtime, and from 9.0 to 19.9% in the case of paid overtime. [...] This is the opposite of the pattern found among the rest of the employed labour force, and most likely is a result of the high rate of unionization among RNs, where rules governing hours of work and overtime compensation are clearly specified within collective agreements.