Criticality Ratio is a term used in construction schedules to represent the percentage or the number of critical activities in a schedule. This ratio can indicate if the schedule is missing logic or an aggressive one.
Generally, a construction schedule with a ratio of 5% or less indicates an illogical schedule. Alternatively, a schedule with a ratio of 20% or more indicates an aggressive schedule.
In this article, we will understand in brief what is a critical path, define critical activities in a schedule, and calculate the criticality ratio.
What is Critical Path
Critical Path is the longest path from start to finish. It calculates the minimum time needed to complete the project. The tasks on this path are called critical activities, and if any delay occurs in these tasks, the project finish date will slip. These tasks are critical if their Total Float is zero.
Here’s how to calculate a critical path using Activity on Node.
Calculate Total Float using either formula:
Total Float = Late Start – Early Start
Total Float = Late Finish – Early Finish
In the above example:
- Critical Path starts with activity “A” through “B” , “E” , “G”, “H”.
To Calculate Criticality Ratio: Divide the number of critical activities (5) over the total number of activities (8) = 63%.
Criticality Ratio in Primavera (P6) Software
Now, Let’s walk through how to calculate this ratio in P6.
- The following illustration depicts an AON network as visualized in Primavera.
- Schedule the project by clicking on “schedule” or pressing “F9” on keyboard. Then click on Schedule.
- Do Step number 2 again and click on View Log.
- Open the log to find the total number of activities under Statistics and the number of critical activities under Exceptions.
- Divide the number of critical activities over the total number of activities to get 63%.
Calculating Criticality Ratio in Schedule Updates
Let’s say, this schedule will be updated, and the schedule is subject to change as the project progresses. Some activities will be finished early, while others may be delayed, affecting the critical path.
As per the above Example, the status for this update is as follows:
2 activities are completed and 1 activity is in progress from total 8 activities .
The formula here is different. Instead of dividing critical activities by the total number, we’ll divide these critical activities by the remaining activities.
Remaining Activities = Not Started Activities + In Progress Activities
Criticality Ratio = (Critical Activities) / (Not Started + In Progress Activities) = 4 / (5 + 1) = 67%.
Summary
The Criticality Ratio assesses the balance of critical activities in a construction schedule. A low ratio (<5%) suggests potential schedule issues, while a high ratio (>20%) indicates a tight schedule. Understanding critical paths and activities is key to calculating this ratio.
By knowing this ratio, the scheduler can identify potential issues, improve schedule accuracy, and facilitate the decision making.
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