Part of the project manager’s job is to help the business understand the reality of what the project will take in terms of the time and cost to produce a particular result (i.e., scope and quality). Just like when you shop for a new car, you may think you want all the best features and top quality, but when you see the price tag, you (the customer) may decide that keeping cost below a certain limit is more important than having all those features. Therefore, you decide that less power and less luxury can suit you perfectly fine. You (the customer) adjust your requirements accordingly.
During the course of the project, the project manager must often make decisions based on achieving or maintaining the right balance of scope (what is to be created and its level of quality), the schedule (time), and the cost (resources). This must be done based on the information known to the project.
Base your decisions and actions as much as possible on agreed-to requirements and known facts. Beware of assumptions that may be wrong. Beware of generalizations about what is required. Beware of wishful thinking. Assumptions, generalizations, and wishful thinking are invitations for project failure – goals not met, schedules missed, and budget overruns.
The project manager and team must understand the scope (requirements), people and resources allocated to the project, the work required, and then develop a project plan that you collectively agree will work. Planning does require estimating and making projections, but the more that you know about any constraints on the project and about the requirements for what is to be produced, the better the planning will be.
The ideal project situation is when, during the initial planning stage, scope is well-defined and risks are low, thus leading to projections of time and cost that have a high probability of being right. (Risks by the way are any factors, known or unknown, that can impact the project.) The challenge, though, comes in the less-than-ideal cases when scope and associated risks are not well-defined at an early stage and require significant analysis or discovery.
These kinds of scope/risk problems occur especially when new technology is being used in product development, implementation, or both. Many other conditions, however – such as when the business itself is changing or when the project is high-risk by its nature – can cause difficulty in managing scope, risks, external dependencies, schedule, and cost.
Most projects do not have scope perfectly defined and understood. Most projects, especially large projects, have substantial amounts of risk. So base the planning and decisions, throughout the course of the project, as much as possible on reality and facts. The probability of success goes up when you avoid false assumptions and guessing, and instead make the plans and decisions based on reliable information.