PERT Estimation – Cool Project Management Software Feature

Written by Gaurav Kheterpal

Some believe project estimation is an art. Others argue that it’s a science based on well-known principles. While there’s no consensus on whether estimation is an art or science, there’s no denying the fact that it’s one of the challenging aspects of project management.

Software projects often employ a number of estimation techniques including Cocomo, Function Point, Program Evaluation and Review Technique (PERT) etc. Each of these estimation techniques has its pros and cons and it’s hard to pick a technique which works equally well for all projects. This post discusses the PERT estimation technique in detail.

Cool Feature – PERT Estimation

Wikipedia defines PERT as a method to analyze the involved tasks in completing a given project, especially the time needed to complete each task, and identifying the minimum time needed to complete the total project. PERT owes its origin to the 1950s when U.S. navy used this method to put the Polaris Fleet Ballistic Missile Project on schedule.

PERT Conventions & Terminologies

The PERT estimation method heavily uses the following conventions and terminologies.

PERT Charts – Simply put, these are visual tools which aid the decision making process.

PERT Events – These are logical milestones which mark the beginning or end of one or more project activities.

PERT Activities – These are actual project tasks which consume time and resources.

Optimistic Time – Best-case estimate of time to complete a task, referred as O

Pessimistic Time – Worst-case estimate time to complete a task, referred as P

Most Likely Time – Normal estimated time to complete a task, referred as M

Expected Time - Best estimate to complete a task, taking project risks into account. Expected time is calculated using the formula

TE = (O + 4M + P) / 6

Slack – Bearable delay in individual tasks which does not delay subsequent tasks
Critical Path – Longest project path from starting activity to ending activity. All activities on critical path have zero slack times.

PERT Estimation

Applying a PERT to a software project involves generating the following three estimates

Optimistic Time
Pessimistic Time
Most Likely Time

These are then used to calculate the weighted-average “Expected Time”. Though it sounds complex, the underlying principle is simple - a single estimate can give us a false sense of precision, so – the more, the merrier!

As with any other estimation technique, PERT starts off with an estimation of the time required to complete each activity. A Gantt chart can then be used to list the activities in a chronological manner. This helps map PERT estimates for subsequent stages of project, taking various activities and their dependencies into consideration. A network activity diagram can also be used with each event representing a node. PERT strictly follows the precedence rule - Before any activity can begin, all its predecessor activities must have been completed.

PERT Advantages

PERT offers the following advantages

Facilitates identification of critical path, thereby reducing overall project risk
Explicitly defines dependencies, thereby aiding in more efficient scheduling
Helps arrive at realistic start/ stop dates for project activities
Added flexibility of fast tracking/ slowing down the project as required

PERT Limitations

Like any other estimation technique, PERT is a mixed bag with the following limitations.

Complexities arise as number of activities and dependencies increase
Lacks objective criteria for defining initial optimistic and pessimistic estimates
May lead to under-estimation of actual project time in some cases

PERT Estimation Tools

There’s a host of PERT estimation tools available – both free and paid. We’ve been particularly impressed with LiquidPlanner, an online project management tool which utilizes PERT concepts to manage change and uncertainty in projects.

LiquidPlanner Scheduling

PERT uses multiple estimates to overcome the traditional project scheduling limitation of relying on single-point estimates for task scheduling. LiquidPlanner takes the PERT notion a step further by using ranged estimates that allow for best- and worst-case scenarios, thereby capturing inevitable uncertainty.

Further, LiquidPlanner relies on probabilistic scheduling to determine the critical path and adjust to schedule variances as and when required. It’s impossible to completely eradicate uncertainty from a project but the deadly combo of probabilistic scheduling and PERT helps minimize the “unknowns”.

The ranged estimates are used to capture the uncertainty at the task level and at the overall project level. Though complex math is widely regarded as the biggest criticism of PERT, LiquidPlanner abstracts managers from these details, letting them focus on more important project tasks.

To know more about LiquidPlanner scheduling, click here.

Conclusion

Uncertainty is the only certainty in a project. If you are looking for an efficient PERT estimation tool, we recommend in this project management software review that you explore LiquidPlanner.