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I have a question about communication channels that is talking about person-month but I don't understand what is a person-month. Does it mean this many persons? or this many months?

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Based on statistical data in a company, communication path between two persons consumes 5% of each person’s time (during a 8-hour work day). For a project that requires 12 persons-months of development time, how many people will be needed to finish the project in four-months if (a) the democratic team structure is used and (b) the chief-programmer team structure is used. If the team consists of four persons, what is the difference in the completion time for a team using the democratic structure and a team using the chief-programmer structure?

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Person-month is politically correct synonym for Man-month.

It's mean amount of work performed by the average worker in one month.

So, if:

  • project requires 12 persons-months of development time
  • all team members do only pure development activity (i.e. they are telepaths and they don't need to spend time for communication with each other). [note: this is not your case. In your case developers spend some time (5%) for communication.]

than:

  • 4 developers will spend 3 months for 12 persons-months project.

ProjectScopeInPersonsMonths / NumberOfDevelopers = NumberOfRequiredMonthsForProject

  • 4 months required for 3 developers to finish 12 persons-months project.

ProjectScopeInPersonsMonths / NumberOfMonths = NumberOfRequiredDevelopersForProject

P.S. This is a wiki article about this term.

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Not to be pedantic, but the correct way to write the term would be person-month, or man-month. The distinction is that person/month implies units of people per month. This is inaccurate.

A person-month is a unit of work that's calculated by multiplying the number of persons by the number of months they work. So a team with three developers working on a task for two months has expended 6 person-months of labor. A person-month is equivalent to hours of labor (160 hours is a common value), but work is often expressed in person-month units because it’s easier to get a feel for how many people might be needed to finish a body of work in a fixed time, or conversely how long it might take a fixed group of people to finish a task.

However, the term man-month been broadly criticized when used as a predictive metric, most famously by The Mythical Man-Month, which makes a strong argument that man-months are not fungible in this naïve manner. That said, it’s still a useful unit of work.

In calculus terms, a person month is calculated by taking the integral of staffing as a function of time between two dates.

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    Yes, of course you are right about writing of "person-month". I fixed my answer. Thx.
    – Sergey Kudryavtsev
    Commented May 13, 2015 at 14:08
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Person month is a measurement unit for effort in software engineering. 1 person month means effort put by a person in one month. But 100 person does not mean, work effort put by 100 person in one month or 1 person in 100 months. As requirement of staff varies time to time in the development so there is not constant no of people is there to work. It is calculated from the graph(calculate area under time axis) between no of people working and time(in month). You can find more detail in COCOMO model.

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