Maths › Decision Mathematics 1 › Critical path analysis
Critical path analysis
Some jobs can wait and some cannot. A forward pass finds the earliest anything can happen, a backward pass finds the latest it may, and the jobs where those agree decide how long the whole project takes.
Builds on Shortest paths: Dijkstra and Floyd and Algorithms, sorting and bin packing.
IN THIS TOPIC
- Draw an activity network from a precedence table, using dummies where needed.
- Carry out the forward and backward passes to find event times.
- Identify the critical activities and the project duration.
COMMON MISCONCEPTION
The critical path is the route through the network with the most activities on it.
From table to network
This paper uses activity on arc. Each activity is an arrow and the circles between them are events. A precedence table lists, for each activity, only its immediate predecessors, and the network must reproduce exactly those dependencies and no others.
Where two activities share some but not all of their predecessors, a dummy arrow of zero duration carries the dependency you want without implying the one you do not. A dummy is also needed when two activities would otherwise share both their start and their end events, since the notation cannot tell such a pair apart.
WORKED EXAMPLE
Reading the dependencies
A project has activities A (3 days), B (5), C (4, after A), D (2, after A), E (6, after B and C), F (3, after D) and G (2, after E and F).
A and B have no predecessors, so both start at time zero.
C and D both wait for A, and E waits for both B and C, which is where a dummy may be needed depending on the drawing.
G waits for E and F, so it cannot start until both branches have finished.
Forward and backward
The forward pass gives each event its earliest time, taking the largest of the earliest finishes of the activities leading into it. The backward pass starts at the end and gives each event its latest time, taking the smallest of the latest starts of the activities leading out.
An activity is critical when its earliest and latest start agree, so any delay to it delays the whole project. The critical activities form a continuous path from start to finish whose durations sum to the project duration. Length in days decides it; the number of activities on a path decides nothing.
GUIDED PRACTICE
The passes in full
For the project above, carry out both passes and state the critical path and the project duration.
Show the working
Forward: A finishes at 3, B at 5, C at 7, D at 5, E at max(5, 7) + 6 = 13, F at 8, G at max(13, 8) + 2 = 15.
Backward from 15: G starts at 13, so E must finish by 13 and start at 7; F may start as late as 10; C must finish by 7.
Critical activities, where early and late starts agree: A, C, E, G.
3 + 4 + 6 + 2 = 15 days, matching the forward pass, which is the check to make.
ASSESSMENT FOCUS
- Copy the durations onto the arcs before starting the forward pass.
- Take the largest value on the way forward and the smallest on the way back.
- Check that the critical activities form one unbroken path and that their durations total the project time.
- Use a dummy whenever two activities share some but not all predecessors, and say why.
- A project can have more than one critical path. Do not stop looking once you have found one.
CHECK YOURSELF
An activity has earliest start 6, duration 4, and latest finish 13. Is it critical?
Show a hint
Compare the latest start with the earliest start.
Show the answer
The latest start is 13 − 4 = 9, which is later than the earliest start of 6, so the activity has 3 days of float and is not critical.
The forward pass takes the largest incoming finish and the backward pass the smallest outgoing start.
An activity is critical when its earliest and latest starts agree; the critical activities form one path whose durations total the project duration.
WORKBOOK
Printable practice for this topic: original exam-style questions with room to work, and a fully worked answer book. Free to use; please do not redistribute or sell.
Or read them with their worked answers on the critical path analysis questions page.
CHECK YOUR PROGRESS
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- Draw an activity network from a precedence table, using dummies where needed.
- Carry out the forward and backward passes to find event times.
- Identify the critical activities and the project duration.
Open the full revision checklist to see every objective in the course in one place.