The project manager is the person with the responsibility to successfully plan and execute the project. He is the person who must create the schedule and believe in it. Even if the schedule was developed by someone else, the project manager must review, modify and own the timeline and the deliverables to be produced. Otherwise, it is too easy for the project manager to opt out of the responsibility by saying that he cannot be held accountable for a schedule that he did not create.
That being said, the project manager does not always have the expertise to build the plan entirely on his own. There are two main approaches for gathering the information required to complete the schedule.
Create a draft and circulate to stakeholders
In this approach, the project manager creates an initial draft of the schedule. This will be a best guess draft of the work that needs to be completed during the project. When the draft is completed, it is circulated to the stakeholders for feedback. The stakeholders can include subject matter experts, or people that have knowledge in certain aspects of the project.
During the review process, work is added, changed or deleted. The project manager takes the feedback and incorporates it into the schedule, which is then used going forward in the project. This approach results in a well-developed schedule and provides opportunities for feedback and buy-in from the stakeholders.
There are two potential risks in this approach. First, the people providing input may not be fully engaged in the project yet. This may result in a lack of focus on the schedule. Second, if the schedule is detailed and lengthy, some of the stakeholders probably will not be able to understand it. This may result in some confusion about how their knowledge of the work integrates with the rest of the schedule. In this case, perhaps a higher-level schedule needs to be created with only summary tasks or milestones.
Build the WBS and schedule through direct stakeholder involvement
In this approach, the schedule is built through one or more live sessions with the major stakeholders and experts. All the stakeholders participate in a facilitated session for one or more days to gain consensus on what needs to be done. If the project is large, you may need to meet with the major stakeholders in groups. For instance, you may have facilitated sessions with each functional department. Each department has a specific way of viewing the project, but a complete schedule can be generated by consolidating the various session results.
This approach has the advantage of having active engagement and participation from the stakeholders. They should have complete buy-in to the work that needs to be done and to their role. This technique might or might not take longer and be more labor intensive than the first option, depending on how many sessions need to be run and how soon the session results can be sent back to the session participants for validation.
In both of these approaches, the project manager will end up with a schedule that was created with input from experts. The project manager will then own the schedule during the project.