Estimating Timelines When Activity Durations Are Uncertain

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Estimating Timelines When Activity Durations Are Uncertain

Software Engineering · Chapter 14: Project Estimation, Configuration, and Change ControlSection 14.1 · lesson 86 of 1037 min · free with a free account

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In this lesson

Making uncertainty explicit, instead of hiding it in 1 number.

  • Explain what a single-point estimate hides
  • Calculate a three-point (PERT) expected duration
  • Describe size-based estimation: lines of code and function points
  • Choose, and combine, the right approach

Transcript

Everything said in the video, section by section (839 words). Read along, or search it.

Project Estimation, Configuration, and Change Control

Estimating Timelines When Activity Durations Are Uncertain

This will take ten days. It sounds precise, but it usually is not. This section covers techniques that make the uncertainty in an estimate explicit, producing an estimate you can actually defend.

A Guess Dressed as a Fact

"This Will Take 10 Days"

A single-point estimate is often a guess, dressed up as a fact. It presents one number as though it were settled, with no sense of how much confidence to place in it. Most estimates are genuinely uncertain. The real duration could reasonably fall anywhere across a range. And a single figure hides exactly how wide that range is. The techniques here exist to make that uncertainty explicit and systematic, rather than pretending it is not there.

Three-Point (PERT) Estimation

3 Numbers Instead of 1

Three-point estimation asks for three numbers instead of one. An optimistic estimate, if everything goes right. A pessimistic estimate, if everything that could reasonably go wrong does. And a most likely estimate, which is the realistic middle expectation. These combine into a single weighted expected value. It is the optimistic figure, plus four times the most likely, plus the pessimistic, all divided by six. So the most likely figure gets four times the weight of either extreme.

Worked Example

O = 4, M = 6, P = 14

Here is an example. A task has an optimistic estimate of four days, a most likely estimate of six days, and a pessimistic estimate of fourteen days. Four, plus four times six, plus fourteen, is forty-two. Divided by six, that gives an expected value of seven days. The wide pessimistic tail nudges the expected value above the most likely figure, even though extreme outcomes are weighted as less probable.

Figure 1

The Shape Behind the Formula

This is a rough sketch of the shape three-point estimation assumes. Most outcomes cluster near the middle, around the most likely figure of six days. But there is a longer tail toward the pessimistic side, stretching out to fourteen. That tail is what pulls the expected value, seven days, above the most likely figure.

Worth Noticing

What the Weights and the Gap Tell You

Two things are worth noticing here. The four-times weighting on the most likely estimate is not arbitrary. It reflects an assumption that extreme outcomes, in either direction, are genuinely less probable than something closer to the middle. And a wide gap between the optimistic and pessimistic figures is useful information in itself. It flags a task that is still genuinely uncertain, and worth watching closely.

Estimating From Size

Size First, Then Effort

A different family of techniques estimates size first, and derives effort from it. Effort is roughly the size, divided by the team's historical productivity rate. A lines-of-code estimate sizes the work by code volume, often by comparison to similar past features, then divides by lines per day. It is simple, but crude. Code volume varies enormously by language and style, and does not capture problem complexity. A function point estimate measures functionality from the user's perspective, counting inputs, outputs, and data files. It stays independent of language, but is more involved to count.

Table 1

3 Techniques Compared

Here are the three techniques side by side. Three-point estimation measures a range of possible durations for one activity. It fits when the activity is understood, but its duration is uncertain. Lines of code estimates code volume, converted through team productivity. It fits when the team has reliable history, in a familiar language and domain. Function points estimate functionality, converted the same way. They fit when comparing size across languages, or benchmarking productivity over time.

Choosing an Approach

Not Either-Or

These approaches are not mutually exclusive. Size-based techniques fit estimating overall effort for a larger body of work, often before individual activities are even worked out. Three-point estimation fits a specific activity whose scope is understood, but whose duration carries real uncertainty. So a project might size its overall scope early with function points, then apply three-point estimates to the activities that turn out most uncertain. Either way, an estimate is a starting point, not a fixed promise. It should be revisited as more is learned.

Apply It

O = 2, M = 3, P = 10

Let's apply that. An activity has an optimistic estimate of two days, a most likely estimate of three, and a pessimistic estimate of ten. What is its expected duration? Two, plus four times three, plus ten, is twenty-four. Divided by six, that is four days. And the wide spread, from two to ten, flags this activity as one worth watching closely.

Key Terms

The Vocabulary of This Section

Let's lock in the key terms. Three-point, or PERT, estimation combines optimistic, most likely, and pessimistic duration estimates into one weighted expected value. The most likely estimate is the realistic middle expectation. It is weighted four times as heavily as either extreme. A function point is a size measure based on counted functionality, such as inputs and outputs, aimed at staying independent of programming language. And lines-of-code estimation converts an estimated code volume into effort, using a team's historical productivity rate.

Key Takeaways

What to Carry Forward

To pull this together. A single-point estimate hides the range of what could realistically happen. Three-point estimation's expected value is the optimistic figure, plus four times the most likely, plus the pessimistic, divided by six. A wide gap between the optimistic and pessimistic figures flags genuine uncertainty. Size-based estimates divide an estimated size by historical productivity. And the approaches can be combined, with every estimate revisited as more is learned.

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FAQ

Questions students ask us

About Estimating Timelines When Activity Durations Are Uncertain

What is three-point estimation in project management?
Three-point, or PERT, estimation replaces a single guess with three figures: an optimistic duration, a most likely duration, and a pessimistic duration. These combine into one weighted expected value, with the most likely figure weighted four times as heavily as either extreme. The approach makes uncertainty explicit instead of hiding it behind one number that only sounds precise.
How do you calculate a PERT expected duration?
The PERT expected duration is the optimistic estimate plus four times the most likely estimate plus the pessimistic estimate, all divided by six. For a task with estimates of 4, 6, and 14 days, that is (4 + 24 + 14) / 6 = 7 days. The long pessimistic tail pulls the expected value above the most likely figure.
What is the difference between lines-of-code and function point estimation?
Lines-of-code estimation sizes work by expected code volume, while function point estimation sizes it by functionality seen from the user's perspective, such as inputs, outputs, and data files. Both then divide size by the team's historical productivity to get effort. Lines of code is simpler but crude, varying with language and style; function points stay language-independent but take more work to count.
What does a wide gap between optimistic and pessimistic estimates mean?
A wide gap between optimistic and pessimistic estimates flags an activity that is still genuinely uncertain and worth watching closely. For example, an activity estimated at 2, 3, and 10 days has a PERT expected duration of 4 days, but its spread from 2 to 10 warns that the real outcome could vary a lot. Estimates should be revisited as more is learned.

About the Software Engineering course

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