If you ask a person studying for a PMP exam, what is the most important area in the domain of project management, the correct answer is "communications". After all it has been documented that on average project manager spends up to 90% of his/her time performing communication activities such as conducting meetings, writing project documents and potentially eliciting requirements via one-on-one interviews.
While I wholeheartedly agree with this notion, I tend to think that the domain of communications is a bit too general to be designated as "the most important thing in project management". I would like to offer to look at this issue from a slightly different perspective:
If you were allowed to write just one project document on your project, which one would you choose?
My answer to this question is the “Requirements Specifications Document”. While some of you may vehemently disagree with me and mention Project Charter and Project Plan. But let us just think about this for a while. The domain of project management is divided into ten knowledge areas (see Figure 1)
Let us examine the impact of scope on other knowledge areas:
Time Management: If you don't have a detailed requirements document can you develop a work breakdown structure and create a network diagram (project schedule)?
Example: The project can take way longer if the customer decides to add a “swimming pool” feature to his villa design.
Budget Management: Can you create a detailed project budget - including human resources required and expenses on equipment and materials - until you have a baselined requirements document?
Example: Will the customer choose a $5-per-square-foot type of flooring or a $150-per-square-foot one?
Quality Management: Can you define the acceptable quality levels and tolerances until you know all of features of the product?