Critical Path Analysis — Edexcel A-Level Business
Test yourself on Critical Path Analysis with PEARSON EDEXCEL A-Level practice questions.
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Critical Path Analysis explained
A project is broken into activities, each given a duration and a list of what must finish before it can begin, and those dependencies are drawn as a network of nodes joined by arrows.
Read the full explanation
The longest route through that network is the critical path, and since every activity on it must run to time, its length is the shortest the whole project can possibly take. The point is management rather than decoration: it tells a housebuilder when to have roof trusses delivered so cash is not tied up in stock, which activities can safely slip, and where to add labour if the deadline is brought forward. It also supplies a yardstick for control once work starts, because a critical activity running late means the completion date has already moved.
b) Complete and interpret simple networks to identify the critical path
Each node carries a number plus two times, each arrow an activity letter with its duration, and nothing may begin until every arrow entering its node has finished. Work forwards adding durations to fill in the earliest times, always taking the largest figure where two arrows meet, then work backwards from the final node subtracting durations and taking the smallest figure where routes rejoin. Nodes whose two times are identical lie on the critical path, and that path is quoted as the run of activity letters, such as A, C, F, H, never as a total. Interpreting it means stating the project duration, naming the activities with no slack whatsoever, and saying what that implies for the staffing, delivery or penalty position described in the case.
c) Calculate: Earliest Start Time; Latest Finish Time; total float
The forward pass gives the soonest an activity can begin: add each preceding duration to the time already at its start node and, where routes converge, take the larger, because the later arrival governs. The backward pass gives the last moment a node can be reached without pushing completion back: subtract each duration from the node ahead and take the smaller where routes split. Spare time on an activity is then the later figure at the node it feeds, minus its own duration, minus the earlier figure at the node it leaves, measured in days or weeks. Two days of slack means a delivery can slip two days before the deadline moves, while nil slack means the activity is critical and a delay of any length costs the project.
d) Limitations of using Critical Path Analysis
Every duration in the network is somebody's estimate, and optimistic estimates produce a confident completion date that one late delivery or a wet fortnight destroys. The method also says nothing about whether the labour and equipment exist to run parallel activities at once, so a plan that looks efficient on paper can demand three teams on the same morning. Large projects generate networks too complex to redraw whenever something changes, and the discipline of protecting the path can push managers to defend the deadline at the cost of quality or of staff goodwill. Its value rests entirely on managers acting on what it shows, so judgement usually turns on the quality of the project data and the volatility of the environment the firm works in.
Your focus
- a) Nature and purpose of Critical Path Analysis
- b) Complete and interpret simple networks to identify the critical path
- c) Calculate: Earliest Start Time; Latest Finish Time; total float
Show all 4 objectives
- d) Limitations of using Critical Path Analysis
Critical Path Analysis exam tips
Marking Points
- Defining the critical path as the longest route through the network, which is therefore the minimum possible project duration.
- Explaining a use in the case, such as ordering materials just in time for the activity that needs them, cutting holding costs and easing cash flow.
- Recognising that activities with spare time can be delayed without moving the end date, so labour and machinery can be switched to critical work.
- Linking the network to monitoring and control while the project runs, not only to planning it beforehand.
- A completed forward pass that takes the highest value where several activities converge on one node.
- A backward pass worked from the final node that takes the lowest value where the network branches.
- Identifying the critical path as the activities whose nodes share identical times and whose spare time is nil, and naming them in order.
- Stating the resulting project duration in the units given, with a comment on what it means for the deadline in the case.
- Taking the largest value where activities converge on the forward pass and the smallest where they diverge on the backward pass, with a reason.
- Applying total float as the latest figure at the following node, less the activity duration, less the earliest figure at the preceding node, showing the working.
- Giving the answer in the time units used in the network and interpreting it, for example that a supplier could be two weeks late without harming the completion date.
- Confirming that an activity with no spare time lies on the critical path, which is the check on the rest of the network.
- Challenging the estimated durations and naming a source of disruption in the case, such as weather on a construction site or a single supplier of one component.
- Explaining that the network assumes resources are available for every parallel activity, which may be untrue for a small firm with one crew.
- Recognising that the analysis only plans, so any time saved depends on managers monitoring progress and acting when a critical activity slips.
- Weighing the limitations against the benefits, for example that even an imperfect network still improves ordering and cash flow.
Examiner Tips
- 💡Short explain questions want the purpose, so give a use with a consequence for cost, cash flow or the deadline rather than a definition alone.
- 💡The phrase just in time is worth using, because the link between the network and inventory costs is a standard mark.
- 💡In longer questions tie the project deadline to a contract penalty or a launch date named in the extract.
- 💡Marks are split between completing the network and using it, so fill in every node even if one calculation defeats you.
- 💡Check yourself by adding the durations along the path you have named and comparing the total with the time at the final node.
- 💡Expect a follow up on the effect of one activity overrunning, where only an activity on the path delays the whole project.
- 💡These are short calculate questions, often two marks each, where correct method with one arithmetic slip still scores, so write the subtraction out.
- 💡Almost every float question is followed by an assess question on whether a delay matters, so keep your figure to argue from.
- 💡Label the halves of each node consistently, because the examiner marks what sits in the earliest and latest boxes.
- 💡Limitations here usually sit inside a high tariff evaluation, so build around two developed points each with a counterargument.
- 💡Refer back to the network you completed earlier in the paper, because applied evaluation outscores generic criticism every time.
- 💡Finish on a condition, such as the effort being worthwhile only where penalty clauses make a missed deadline expensive.
Common Mistakes
- Describing the critical path as the quickest or shortest route, when it is the longest route and therefore the one with no spare time at all.
- Assuming the network shortens a project by itself, when any time saved comes from the decisions managers take after reading it.
- Treating every activity as critical, which loses the whole point of spare time and of moving resources onto the path that matters.
- Taking the first or smallest number on the forward pass instead of the largest, which produces a project duration too short to be possible.
- Quoting the critical path as a number of days rather than as the sequence of activities.
- Drawing an activity as starting before a preceding activity has finished, which breaks the dependency table given in the question.
- Subtracting in the wrong order, which produces a negative amount of spare time and a candidate who then ignores the impossible answer.
- Reading the later figure from the wrong node, typically the node the activity leaves rather than the node it runs into.
- Confusing total float with free float and then claiming several activities on one path can each be delayed by their own float at the same time, when the spare time is shared along that path.
- Stating that it is complicated without giving the consequence, which is that it absorbs management time and is therefore rarely updated.
- Saying it ignores cost, when the sharper point is that it ignores the cost of the resources needed to run activities in parallel and the cost of shortening an activity.
- Concluding that the business should abandon the technique rather than judging the conditions under which it earns its keep.