No matter how experienced a consultant is in sizing UCD projects, (s)he is continually faced with the challenge of effectively sizing a UCD project so that the actual end time spent on the various activities reflects what was originally proposed. The process of estimating the time it will take to complete any given phase of a UCD project, not to mention the entire project, may be affected by many variables, including the following:

• The iterative nature of the UCD process.
• Budgets.
• The extent to which the client understands and "buys into" the UCD design process.
• Political issues.
• "Scope creep" (the tendency for a project’s scope to grow through the addition of requirements not originally understood, considered, or planned for).
• Ill-defined requirements.
• The makeup of the team, which may vary or change mid-stream.
• Other factors more important than the actual scope and time required to complete the project.
• Insufficient or unavailable data at time of project sizing.

Further, sizing is often an iterative process. Often a consultant will need to "take the pulse" of the project at various points, each time providing a sense of time and resource requirements. Thus, sizing a project is a critical yet difficult activity.

Consultants and others responsible for sizing UCD projects have various strategies for providing clients estimates for UCD work. This paper will focus on two aspects of this topic: 1) Identification of why it is so difficult to size a usability/UCD effort, and 2) identification of strategies, including their respective pros and cons. Throughout this paper, we will use the term "consultant" inclusively to refer to those who provide usability and UCD services for clients (external consultants), as well as those who do so for groups within their own organization (internal consultants).