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A man-hour or person-hour is the amount of work performed by an average worker in one hour.[1][2] It is used in written "estimates" for estimation of the total amount of uninterrupted labour required to perform a task. For example, researching and writing a college paper might require twenty man-hours. Preparing a family banquet from scratch might require ten man-hours.
Man-hours do not take account of the breaks that people generally require from work, e.g. for rest, eating, and other bodily functions. They only count pure labour. Managers count the man-hours and add break time to estimate the amount of time a task will actually take to complete. Thus, while one college course's written paper might require twenty man-hours to carry out, it almost certainly will not get done in twenty consecutive hours. Its progress will be interrupted by work for other courses, meals, sleep, and other distractions.
The advantage of the man-hour concept is that it can be used to estimate the impact of staff changes on the amount of time required for a task. This is done by dividing the number of man-hours by the number of workers available.
This is, of course, appropriate to certain types of activities. It is of most use when considering 'piece-work', where the activity being managed consists of discrete activities having simple dependencies, and where other factors can be neglected. So, adding another man to a packaging team will increase the output of that team in a predictable manner. In transport industry, this concept is superseded by passenger-mile and tonne-mile for better costing accuracy.
In reality, other factors intervene to reduce the simplicity of this model. If some elements of the task have a natural timespan, adding more staff will have a reduced effect: although having two chefs will double the speed of some elements of food preparation, they roast a chicken no faster than one chef. Some tasks also have a natural number of staff associated with them: the time to chop the vegetables will be halved with the addition of the second chef, but the time to carve the chicken will remain the same.
Another example is the old adage, "Just because a woman can make a baby in 9 months, it does not follow that 9 women can make a baby in one month." This adage is oft cited in systems development to try and justify the belief that adding more staff to a project does not guarantee it will get done quicker.
Another problem with this model, as Fred Brooks noted, is that organization, training, and co-ordination activities could more than outweigh the potential benefits of adding extra staff to work on a task, especially if considered only over a shorter time period.
The similar concept of a man-day, man-week, man-month, or man-year[3][4] is used on very large projects. It is the amount of work performed by an average worker during one day, week, month, or year, respectively. The number of hours worked by an individual during a year varies greatly according to cultural norm(s) and economics. The average annual hours actually worked per person in employment as reported by OECD countries in 2007, for example, ranged from a minimum of 1389 hours (Netherlands) to a maximum of 2316 hours (South Korea).[5]
Man-years: