Reading a precedence table
What critical path analysis is
- Critical path analysis (CPA) is a method for planning and scheduling a project built from many separate activities.
- An activity is a single task that takes a known duration and may depend on other tasks being finished first.
- CPA answers three key questions:
- How long will the whole project take at the earliest?
- Which activities must not be delayed, or the whole project slips?
- Where is there spare time that could absorb a hold-up?
- It is used to manage real projects — a building site, an event build, a marae kitchen upgrade — where tasks overlap and share deadlines.
What a precedence table shows
- A precedence table (also called a dependency table) lists every activity with two facts:
- its immediate predecessor(s) — the activities that must finish immediately before it can start,
- its duration — how long it takes.
- Read each row as a sentence: "Activity D cannot start until B and C are both finished, and it takes 5 days."
- A dash (—) in the predecessor column means the activity has no predecessor — it can start at the very beginning.
Immediate predecessors, not all predecessors
- List only the immediate predecessor — the task directly before, not every task earlier in the chain.
- If comes before and comes before , then 's immediate predecessor is only, not and .
- is still a predecessor of , but it is an indirect one — the network carries that link through .
A worked precedence table
- For the community hall build the table reads:
| Activity | Description | Immediate predecessor(s) | Duration (days) |
|---|---|---|---|
| A | Clear and level the site | — | 3 |
| B | Pour the foundations | A | 4 |
| C | Order and deliver fittings | — | 2 |
| D | Build the frame | B, C | 5 |
| E | Install the roof | D | 3 |
| F | Rough-in the wiring | D | 2 |
| G | Interior fit-out | E, F | 4 |
- A and C start at the beginning. D waits for both B and C. G waits for both E and F.
A Kāpiti event crew is putting up a stage. They must: build the platform (2 days); before that, lay and level the base (3 days); once the platform is up they can both bolt on the handrails (1 day) and rig the lighting truss (2 days); the sound check (1 day) can only happen once the handrails and the truss are both done. Write the precedence table.
Step 1 — Name each activity
Give every task a letter and its duration:
- P — lay and level the base (3)
- Q — build the platform (2)
- R — bolt on handrails (1)
- S — rig the lighting truss (2)
- T — sound check (1)
Step 2 — Find each immediate predecessor
Read the dependencies from the description:
- P starts first, so it has no predecessor.
- Q needs the base, so its predecessor is P.
- R and S both need the platform, so each has predecessor Q.
- T needs the handrails and the truss, so its predecessors are R, S.
Step 3 — Write the table
| Activity | Immediate predecessor(s) | Duration (days) |
|---|---|---|
| P | — | 3 |
| Q | P | 2 |
| R | Q | 1 |
| S | Q | 2 |
| T | R, S | 1 |
Test yourself
Practice by grade
One question each at Achieved, Merit and Excellence. Have a go, then compare with the model answer.
In a precedence table, activity M has 'K, L' in the immediate-predecessor column and a duration of 6. Explain in words what this row tells the project planner.
A student writes that activity D depends on 'A, B, C'. The table already shows B depends on A, and C depends on A. Rewrite D's immediate predecessors correctly and justify the change.
A café fit-out has activities J, K, L, M. K and L can each start only after J. M can start only after both K and L. The owner claims 'M depends on J as well, so J should be listed for M.' Evaluate this claim and state the general rule it breaks.