How to Determine the Critical Path & Expected Completion Time

Getting a large project completed on schedule requires advanced planning and attention to detail. Diagramming the project workflow and determining the critical path gives you the expected length that everything will take from inception to completion. For that to be accurate, you need to do a thorough job estimating how long each task will take.

To determine the critical path, divide each project into smaller tasks and note dependencies between them. List the related tasks in the order they must take place and note the earliest date they can start and finish without causing the next activity to be delayed. You’ll find that most projects have multiple paths, since some required activities can be run in parallel. For example, if you’re opening a restaurant, you can be negotiating with produce suppliers at the same time the electricians are wiring your dining area, so those would fall under separate paths. Write out the relationships between tasks as a network diagram, and find the path with the longest duration. That’s the critical path, since any delay in that path will delay the completion of the project.

To estimate how long it will take to complete a task, you need an estimate of the available resources. Determine how many workers you’ll have, whether they will be employees or contractors, and what their level of expertise is. If this is a project where equipment plays a key role, get that firmed up as well. You don’t want to base your scheduling for a construction project on the assumption of having two cranes only to find out you’re only getting one. Also note any schedule constraints, like whether a holiday will interrupt the work week, or whether a competing project threatens to siphon off some of your staff by a certain date.

Getting an accurate completion time ideally involves the input of the workers in charge of performing the tasks. Balance that feedback against historical data and lessons learned from your past projects, as well as industry averages. This serves as a check against schedule padding, in which someone might offer an overestimation of the required time to provide some wriggle room. Also take into account external factors that could alter the schedule. Besides resource allocation questions, this might include environmental factors, or cultural and information systems differences if you’re working with another company.

Once you’ve gathered the opinions of those on the task and compiled the data, it’s your job to provide the final estimates. There are a number of techniques for doing so. In one-point estimating, you simply provide one time estimate for how long an activity will take. Analogous estimating takes into account prior history and uses that as a predictor of present task length. Parametric estimating relies on the relationship between activity variables to come up with a time, while heuristics simply estimates everything by using a rule of thumb, such as setting up the site always takes up 20 percent of the project time.

The most comprehensive way of estimating completion times is by using the Program Evaluation and Review Technique. PERT uses risk management techniques to account for the possibility that a project may be completed ahead of schedule or suffer unanticipated delays. You come up with three different time lengths: the realistic amount of time it is most likely to take, an optimistic assessment, and a pessimistic assessment. Multiply the realistic time by four, add the positive and negative time scenarios, and divide the result by six. That's the time you'd use for estimating task length when compiling the activities to determine the critical path.