How Much Is Too Much
In Excessive Schedule Pressure, I describe why Project Managers should never excessively try to compress a project schedule. What I failed to do however is define the term "excessive". As a Project Manager, how can you determine if your schedule is aggressive yet realistic, and at what point does it become overly optimistic? How much is too much?
The Efficient Project Schedule
The law of diminishing return explains that every project has a point beyond which each additional resource will be less productive than the previous one. That point is the Efficient Project Schedule (EPS), sometimes referred to as the "nominal" schedule.
The EPS is an aggressive, realistic project schedule that is effective without wasting resources. It should be used when the cost of a project is relatively more important than time to market.
The following mathematical expression allows you to estimate the duration of the EPS:
D = K * E1/3, where
D is the duration of the project
K is a constant (usually 3, but can range between 2 and 4)
E is the estimated effort for the entire project, in person-months.
Imagine a project that was estimated to take 100 person-months to complete. According to the EPS formula, the time required to complete the project is approximately 14 months (D=3 * 1001/3). Given that the overall effort is estimated to take 100 person-months, you need to assign approximately 7 developers to the various development tasks. Assigning less than 7 developers means there's plenty of room for accelerating the project. Assigning more than 7 means you've accelerated the schedule, but not cost-effectively. The additional resources are relatively less productive than the first 7, and you are consequently not getting your biggest bang for your buck.
The Schedule Compression Factor
The Schedule Compression Factor (SCF) explains the degree to which an EPS has been compressed. Its mathematical formula is:
SCF = DRPS/DEPS, where
DRPS is the duration of the rapid project schedule
DEPS is the duration of the efficient project schedule
Most researchers have concluded that it is impossible to have an SCF lower than 70% to 80%. In other words, no matter how many additional people you throw at a project, you will not compress its EPS by more than 20% to 30%.
The Rapid Project Schedule
When speed is more important than cost, Project Managers should plan their project based on the Rapid Project Schedule (RPS).
The law of diminishing return explains that each additional resource beyond a certain point becomes less productive than the previous one. Naturally, the duration of the project will still be reduced if you keep adding resources, but the decline in duration is less significant with each additional person. As capacity increases, marginal production tends toward 0.
Under many circumstances however, time to market is more important than project cost. Consequently, many companies prefer to add one or two additional developers to the project only to be able to compress that schedule by an extra percentage point.
If your project requires approximately 100 person-months of effort, and you divide the tasks among 7 developers, your EPS would last approximately 14 months. You could potentially reduce that schedule to 10 or 11 months by throwing more bodies at the problem, but no matter how many people you add, the schedule cannot be compressed to 9 months or less.
Implications
The implications behind EPS and RPS are very important to understand.
First, you need to realize and accept the fact that a 100 person-month project cannot be delivered in 9 months or less. No matter how much pressure your management team applies, a good Project Manager should never accept to compress such a schedule beyond 10 or 11 months.
Second, keep in mind the law of diminishing return. If we go back to the 100 person-month project, we can assume that the people/duration relationship is pretty much linear until you reach 7 people. It won't be perfectly linear, but the law of diminishing return is negligible. In other words, if you only have only 1 person working on that project, the duration will be 100 months. If you have 2, it will be 50 months. And 25 months for 4 people, and so on.
However, once you reach the EPS, the compression is not linear anymore. The 8th and 9th person might only slightly reduce the duration of the overall schedule. And adding a 10th and 11th person might not accelerate the project at all. The extra overhead and communication required to manage 4 more people might actually slow down the project. Conclusion: Don't expect to reduce the overall duration of the project from 14 months to 12 months by simply adding an 8 th person. Once you reach the EPS point, take it for granted that the decline in duration is less significant with each additional person and that as capacity increases, marginal production tends toward 0.
Finally, this article assumes that the total effort estimated to complete the project is legitimate. If the 100 person-month forecast is realistic, then the rules and formulas presented in this article will help you develop an aggressive, yet realistic project schedule. However, if the 100 person-month baseline is really 125 person-months that you've already compressed with no valid justification, then your project is in jeopardy – plain and simple.
This article was originally published on www.gantthead.com.