PERT – How to accurately estimate the duration of tasks in a project?

Effective project management is not just about controlling costs and progress but also about a realistic approach to time planning, especially when uncertainties are involved. Here, the PERT technique (Program Evaluation and Review Technique) comes to the rescue – an estimation method that takes into account various time scenarios for project tasks.

Link to the calculator: PERT

What is PERT?

PERT is based on the assumption that each task can have three different execution times:

✅ O (Optimistic time) – when everything goes perfectly.
⚙️ R (Realistic time / most probable) – when everything goes normally.
⚠️ P (Pessimistic time) – when difficulties and delays arise.

Instead of relying on a single value, PERT combines them into a coherent formula, allowing for a more accurate prediction of how long it will actually take to complete a task.

📊 Key formulas in PERT

Below we present all the most important formulas used in PERT along with their application and interpretation:

⏱ E – Expected Time

Formula:
E = (O + 4 × R + P) / 6

Description:
This is the main PERT formula. It averages the three times, giving the highest weight (4x) to the realistic time. It allows for a more balanced estimation.

📉 V – Variance of duration

Formula:
V = ((P - O) / 6)²

Description:
A measure of uncertainty – the greater the difference between the pessimistic and optimistic times, the greater the variance.

📐 σ – Standard Deviation

Formula:
σ = (P - O) / 6

Description:
Indicates how much the actual execution time may deviate from the expected time.

🔀 T – Triangular Average

Formula:
T = (O + R + P) / 3

Description:
A simple average of the three values, used in cases of high uncertainty or when all three times are equally likely. Based on triangular distribution.

📊 SD – Standard Deviation (triangle distribution)

Formula:
SD = √((P - O)² / 18) or SD = (P - O) / 4.24

Description:
An alternative way to calculate the deviation, assuming a uniform probability distribution across all values.

🔍 Example of Application

Suppose we are planning a task with the following values:

O = 2 days
R = 5 days
P = 8 days

Calculations:

Expected Time (E):(2 + 4 × 5 + 8) / 6 = 30 / 6 = 5 days
Variance (V):((8 - 2) / 6)² = (1)² = 1
Standard Deviation (σ):(8 - 2) / 6 = 1
Triangular Estimate (T):(2 + 5 + 8) / 3 = 15 / 3 = 5 days
SD for triangular distribution:(8 - 2) / 4.24 ≈ 1.42

✅ Why Use PERT?

📌 Allows for more accurate time estimations than a single value.
🧭 Takes into account uncertainty and variability in the project environment.
📈 Facilitates the identification of high-risk delay tasks.
🔍 Supports realistic scheduling and resource planning.

🧠 Summary

PERT is an indispensable tool in project planning, especially where it is difficult to predict the execution time of tasks accurately. By combining different scenarios, PERT provides more reliable data than traditional methods.

PERT formula - how to accurately estimate the duration of tasks in a project?