while it is desirable for predictive models like weather forecasting models to produce projections that are as accurate as possible the most important feature of a longerterm health forecasting model should be its ability to influence policy makers to do that health expenditure projections not only need to be scientifically sound but must also be considered credible zucchelli et al the credibility of health expenditure forecasting models in turn relies on their validity accuracy tractability and transparency harris et al the validity of a model refers to its ability to address the request of policy makers a model is valid if its structure reflects relevant features of the policy environment and avoids nonessential elements