Estimating Durations

After the project has been divided into tasks and resources assigned to those tasks, it’s time to determine the task durations. This importance of this step should not be taken lightly, because these durations will be used to construct the project schedule.

Just like the tiny o-rings that brought down the space shuttle Challenger, one small underestimation often results in a chain reaction that the project can’t recover from.

The PMBOK

The PMBOK contains a process called “Estimate Activity Durations” which encompasses the tasks involved with preparing task duration estimates.

PMBOK, 5th Edition, Section 6.5, “Estimate Activity Durations”

Estimate Activity Durations is the process of estimating the number of work periods needed to complete individual activities with estimated resources. The key benefit of this process is that it provides the amount of time each activity will take to complete, which is a major input into the Develop Schedule process.

To determine the availability of resources, a project manager consults a document called a resource calendar. This states the availability of a resource, for example,

Mark is at jobsite X from Sep. 1 to Sep. 30

Obviously, the new project being planned cannot use this worker at that time (unless arrangements are made with the other project manager, I suppose). The resource calendar can range from a simple listing of workers’ availability to full calendar within project management software that tracks employees schedules.

On the previous page we listed the three main types of estimates. On this page we will expand them, work through them, and add one more. The four types of estimates are:

Analogous Estimating
Parametric Estimating
Three point Estimating
Expert Judgment

Analogous Estimating

As the name suggests, this is when you make an analogy to the same, or similar, task that has been performed before. This is the best source of information because actual work completed is extremely reliable, even if it requires adjustments. For our log home example, previous projects provide an excellent comparison to determine resources, costs and durations. In fact, many companies have numerous similar projects, and this is the natural starting point. For each task, however, you often have to make some adjustments. Here is a checklist of adjustments:

The task is bigger or smaller than the last one.
Scale the duration up or down by the appropriate amount. For example, the log house has slightly more square footage, or the driveway is longer, or several more windows have been added. This might sound obvious but it’s not the big, obvious things that cause problems for project managers, it’s the little things, and you have to make sure you include enough contingencies for them.
The task needs specialty products or services that the last one didn’t.
In this case, add in the appropriate products and services and proceed as usual. This also sounds obvious but has equally high potential for project overruns. For example, the slightly bigger roof trusses require the next larger size of crane. Or the foundation requires removal of rock instead of clay.
The task is for a slightly different end product.
Determine the difference in materials and/or workmanship and figure out the difference in resources, cost, and time. For example, you could calculate from the plans that the log home requires 20% more timber than the previous one. Then you could estimate that it would take 20% more time in the Wood Work task.

One of the most problematic issues with analogous estimating is that it’s only as reliable as the task the estimate is being taken from. Often you have to have been the project manager of the comparator project, or at least close enough to it to know what happened and how it went. Say, for example the crane broke down and spent four days on site instead of two, you would be double the duration (and price) if you didn’t know about that detail and therefore didn’t account for it.

Also, the productivity of the work is an important consideration. For knowledge workers such as engineers, adding more of them sometimes slows the production and decision making process down instead of speeding it up. Likewise, a very experienced laborer has higher productivity than a less experienced one. Thus, if the previous project used an inexperienced laborer to place the roof trusses, and this time you will be using a more experienced one, the estimate can, and should, be different.

Parametric Estimating

A second common method for estimating task durations requires that the work be drilled down into a unit cost, such as a construction time per square foot of log home. Here are some examples:

Total construction time per square foot of log home.
Pouring time per cubic yard of concrete.
Installation time of carpet per square foot.

In the engineering industry almost everything is done this way, from the engineering time down to the construction materials. If the log home company kept track of the construction time, especially for the individual task of “Wood Work,” together with the square footage of the home, they could eventually arrive at a pretty good parametric estimate for the task duration. If your company has many similar projects and doesn’t do this, it should.

Sometimes a parametric value includes fixed items, such as doors and windows which require a fixed time for installation regardless of how much timber there is to construct. In this case the fixed items can be separated out and added to the overall parametric estimate.

Three-point Estimating

In this method, the estimator determines three numbers:

Optimistic
Most Likely
Pessimistic

The second, “Most Likely” is the average of what the task duration would be if performed many times. It is the “normal” estimate that you might otherwise come up with using the other methods. The other two represent the upper and lower bounds that you are not likely to exceed.

To do this numerically you would assign a confident interval for the upper and lower bounds (optimistic and pessimistic values), such as 90%. Many textbooks use 99% or some other value, the methodology varies. But if you consult previous projects and determine the 90% confidence interval (1 out of 10 projects exceeds) you can run with that.

Generally one of two distributions is used. The triangular distribution is simply an average of the three estimates and the beta distribution is a bit tighter.

Triangular (average) distribution	Beta distribution
t_e = (a + m + b) / 3	t_e = (a + 4m + b) / 6

Where:

t_e = Expected duration
a = Optimistic
m = Most likely
b = Pessimistic

The triangular distribution will skew the results to the value which is far away from the Most Likely value. If you want the estimate to be tighter (closer) to the Most Likely value, use the beta distribution. I don’t believe it should get more complicated than this. You have to decide how tight to the Most Likely value you want to be.

For example, let’s say the expected duration of the task Wood Work is 18 days (m = 18), and you figure there is significant potential for delays. The Optimistic estimate is 15 days (a = 15), but the Pessimistic estimate is 33 days (b = 33). The beta distribution yields:

t_e = (15 + 4 x 18 + 33) / 6 = 20 days

The original estimate was 18 days, but because of the potential of reaching as much as 33 days, the estimate should be increased to 20 days.

Proceeding further down this road, the standard deviation of the task is simply:

σ_te = (b – a) / 6

If you add up all of the durations (∑m) and standard deviations (∑σ) for each task, you will have the total project duration as well as it’s standard deviation. You can draw the following conclusions from this:

The confidence level in one standard deviation, i.e. M ± σ is about 68%.
The confidence level in two standard deviations, i.e. M ± σ is about 95%.
The confidence level in two standard deviations, i.e. M ± σ is about 99.7%.

In the example above, for the Wood work task the standard deviation is (33 – 15) / 6 = 3.0. Therefore, the task duration is:

20 days is the official estimate.
23 days with 68% confidence.
26 days with 95% confidence.
29 days with 99.7% confidence.

Wouldn’t your boss love this?

Just don’t be fooled by fancy statistics if your input data is unreliable. The result is only as good as the input data, in other words, garbage in, garbage out.

Expert Judgment

Professional estimators will tell you that in spite of all the techniques in the book, a technical expert will be the best resource you could ever have. They could be the only resource you need for estimating. Technical experts are notoriously busy because their expertise gets used on many projects. But if you have access to one, whether inside the company or out, you should find a way to use them.

But what if a technical expert contradicts the other methods? For example, let’s say you’re installing carpet and your carpet installer tells you a certain project will take 4 days, but past experience (analagous) is telling your it will take 10 days. How valuable is the expert’s opinion then? It is difficult to give good advice that applies to all situations. You must simply weigh the two methods against each other, considering the underlying data of each method. For example, maybe this carpet installer frequently underestimates duration, or the past experience estimates are skewed by a certain piece of data.

Project management itself does not exist independent of the technical expertise inherent in the project. That is, the project manager or management team must be at least passingly familiar with the technical aspects of the work or the project has little chance of success.

Example Project

To build our log home, we would inspect each resource table from the previous step and determine the amount of time required. In this case I have included contingencies of between 50% and 100%, because the duration needs to include downtime. For example, there might be 3 days of plumbing but the task needs to include the time before they arrive, and I don’t believe they are likely to arrive on time.

Task ID	Name	Durations	Cost
110	Excavation	6 days	$5,850
120	Pour Foundation	10 days	$4,700
210	Wood work	20 days	$60,850
310	Electrical & Plumbing	10 days	$20,260
320	Flooring	8 days	$18,760
330	Finishing	12 days	$16,760
410	Landscaping	14 days	$7,620
TOTAL		80 days	$134,800

Do you think you could build a small log home for $134,800?

The total duration of 80 days at this point doesn’t mean much. A simple addition of task durations does not take into account a potential reduction due to performing of tasks in parallel, nor a potential increase due to the availability of resources. In the next step we will determine the “critical path” so we can potentially shorten the schedule.