A natural part of project control is to periodically compare actual effort and duration for individual tasks with estimated and planned values and then re-estimate, and re-allocate resources when necessary.Normally one of the following techniques for estimating effort and schedule is used, with a preference for decomposition and modeling.
The techniques can either be applied bottom-up or top-down. Bottom-up estimation begins with the lowest-level parts of product or task, and provides estiamates for each. Top-down estimation begins with the overall process or product.
- expert opinion: estimate is made based on experts past experience
- analogy: identifying a similar past project and adjusting
- decomposition: divide and conquer
- models: using a model relating key inputs and effort
The models can be divided into cost models, providing direct estimates of effort or duration, and constraint models, demonstrating the relationship over time between two or more parameters of effort, duration, or staffing level, see Example 12.4 and Figure 12.4.Cost models are often based on empirical data reflecting factors that contribute to overall cost. Their input consist of one primary input (size) and a number of secondary adjustment factors (cost drivers).
One of the models used is
E = aSb wher a, b are parameters that are estimated by regression techniques, see Figure 12.5.