CPM is useful for determining the longest sequence of dependent tasks, but it has several drawbacks. Here's what you should be aware of:
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Rigid Task Dependencies - CPM assumes permanent dependencies when, in practice, teams frequently work in parallel or find ways to dynamically abbreviate jobs.
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Limited Resource Consideration - It does not account for resource limits. You may have an optimum schedule, but if crucial persons or materials are unavailable, delays will occur.
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Unrealistic Float Assumptions - CPM estimates float (slack time), but this assumes that tasks' priorities do not vary, which is not always the case in dynamic contexts.
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Difficulty Handling Uncertainty - Variability in task durations is underappreciated. Unlike PERT, CPM does not take into account best- and worst-case situations.
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Ignoring External Factors - External dependencies (vendor delays, government approvals) can derail even the best-planned CPM strategy.
CPM has a solid basis, but it is most effective when combined with risk analysis and resource management to prevent bottlenecks.