Stressed vs Relaxed CPM Networks – The Official Site for Zümmer

Overview

Analyzing and understanding the distribution of Total Float (TF) values across a CPM network is a simple but revealing health check. As a general rule, CPM networks with a more “Stressed” TF distribution are more sensitive to Project delays and claims than CPM networks with a more “Relaxed” TF distribution.

How to Read the Total Float Distribution Curve

Zümmer’s Total Float Distribution Curve report plots every task’s Total Float value allowing you to evaluate the condition of the CPM network at a glance:

X-axis — the Total Float value, in days.
Y-axis — Percentile of All Tasks: the percentage of tasks that have that Total Float value or less.

Example of a “Stressed” CPM Network

In the “Stressed” CPM Network example shown above, the 1,000 tasks span Total Float values from 0 to 200 days. Reading the curve:

At the 50th percentile, Total Float is about 28 days — half of all tasks have about 28 days of Total Float or less. (Red Arrows)
At the Midpoint of Total Float values (100 days Total Float) – about 82% of all tasks fall at or below the midpoint Total Float value. (Green Arrows)
More than 10% of tasks have zero Total Float and sit on the Critical Path (Yellow Arrow).

Example of a “Relaxed” CPM Network

In the “Relaxed” CPM Network example shown above, the 1,000 tasks span Total Float values from 0 to 200 days. Reading the curve:

At the 50th percentile, Total Float is about 108 days — half of all tasks have about 108 days of Total Float or less. (Red Arrows)
At the Midpoint of Total Float values (100 days) – about 40% of all tasks fall at or below the midpoint Total Float value. (Green Arrows)
Only about 5% of tasks have zero Total Float and sit on the Critical Path (Yellow Arrow).

Interpreting the Curves

All else being equal, Projects with a “stressed” distribution are more likely to have a greater potential for claims and experience delays than those with a “relaxed” distribution. A precise rule of thumb is debatable, but by observation:

Identifying Stressed CPM Networks — curves that are steep on the left side and flat on the right side. In Stressed CPM networks, the red arrows are well inside the green arrow boundary. Also, the percent of activities on the Critical Path is excessive. Typically, 10% or more is considered the high boundary threshold.
Identifying Relaxed CPM Networks — curves that rise smoothly from beginning to end. In Relaxed CPM networks, the green arrows are typically inside the red arrow boundary. Also, the percent of activities on the Critical Path is low. Typically, 5% or below is considered the low boundary threshold.

When to Take Action

A closer look at the schedule’s logic may be warranted when either Stressed or Relaxed conditions exists in the extreme.

Very stressed distributions may signal a “claims-oriented” schedule. Review the overall logic to find relationships that can be removed or revised, thereby increasing overall Total Float.
Very relaxed distributions may signal an insufficiently developed schedule. Review the overall logic to find relationships that can be added or revised, thereby decreasing overall Total Float.