CASE (computer aided software engineering) tools are believed to have played a critical role in improving software productivity and quality by assisting tasks in software development processes since the 1970s. Several parametric software cost models adopt "use of software tools" as one of the environmental factors that affects software development productivity. Several software cost models assess the productivity impacts of CASE tools based only on breadth of tool coverage without considering other productivity dimensions such as degree of integration, tool maturity, and user support. This paper provides an extended set of tool rating scales based on the completeness of tool coverage, the degree of tool integration, and tool maturity/user support. Those scales are used to refine the way in which CASE tools are effectively evaluated within COCOMO (constructive cost model) II. In order to find the best fit of weighting values for the extended set of tool rating scales in the extended research model, a Bayesian approach is adopted to combine two sources of (expert-judged and data-determined) information to increase prediction accuracy. The extended model using the three TOOL rating scales is validated by using the cross-validation methodologies, data splitting, and bootstrapping. This approach can be used to disaggregate other parameters that have significant impacts on software development productivity and to calibrate the best-fit weight values based on data-determined and expert-judged distributions. It results in an increase in the prediction accuracy in software parametric cost estimation models and an improvement in insights on software productivity investments.