A lively discussion is under way at Sunrise Software, where you are a project ma

Question

A lively discussion is under way at Sunrise Software, where you are a project manager. The main question is whether the person-days concept has limitations. In other words, if a task will require 100 person-days, does it matter whether the work is performed by two people in 50 days, five people in 20 days, 10 people in 10 days, or some other combination that adds up to 100? Is Hector correct? If so, what are the limits in the people versus days equation? Taking the concept a step farther, is there an optimum number of people to be assigned to a task? If so,

Explanation / Answer

1) Yes and no.

Example: If you calculate a project with four developers and one (purely managerial) project lead, you basically add 25% overhead to the effort. If you then double the number of developers to reduce the duration to 50%, the overhead percentage might increase or decrease depending on the people involved. There are non-linear effects due to communication and "noise"/cooperation between project participants.

So, the equation "duration = effort / full-time-equivalents" is only a very rough estimate. Additionally, you have to take into account public holidays, vacancies, illness, competing projects, resource priorities, etc. to derive the calender duration from the estimated effort.

2)"Effort" is measured in man-hours (or man-days, man-weeks, man-months, or even man-years). "Duration" is measured in hours, days, weeks, months, or years.

You start by estimating the effort required to complete the project. There are lots and lots of ways to estimate, some better than others, none perfect. (Strong hint: Linear estimators ("ten lines per man per day") do not scale up well.)

Once you have an effort estimate, you have to estimate the duration. This is closely related to constructing a draft (strawman) schedule, and inherently involves decisions on how many people you will put on the project. To a certain extent, headcount can be traded for schedule, but bear in mind that there is still a minimum duration for some things The better estimating methodologies will give you an "optimum" duration and headcount, given an effort estimate, and give you guidance on adjusting the effort estimate if you need a duration significantly longer or shorter than the optimum. (Generally, both cases yield higher efforts, and hence higher cost.)

For some duration estimates, you may have to take into account that you need specialists for some tasks, and they don't grow on trees. If you have three Kalman filters that have to be set up and tuned, and only one Kalman filter expert, you are not going to be able to do those three tasks in parallel.

3)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. Therefore, adding another person 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 adage, "Just because a woman can make a baby in nine months, it does not follow that nine women can make a baby in one month." This adage is often cited in systems development to 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.

4)Hours have an implied precision and tend to be looked at as 100% accurate, all humans understand what an hour is so if you say 10 hours it must be ten hours. To compensate for this the person estimating will build in some "extra time" to compensate for the unknowns. It is just human nature they don

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