Combining classifier systems potentially improves predictive accuracy, but outcomes have proven impossible to predict. Classification most commonly improves when the classifiers are ''sufficiently good'' (generalized as ''ACCURACY'') and ''sufficiently different'' (generalized as ''DIVERSITY''), but the individual and joint quantitative influence of these factors on the final outcome remains unknown. We resolve these issues. Beginning with simulated data, we develop the DIRAC framework (DIVERSITY of Ranks and ACCURACY), which accurately predicts outcome of both score-based fusions originating from exponentially modified Gaussian distributions and rank-based fusions, which are inherently distribution independent. DIRAC was validated using biological dual-energy X-ray absorption and magnetic resonance imaging data. The DIRAC framework is domain independent and has expected utility in far-ranging areas such as clinical biomarker development/personalized medicine, clinical trial enrollment, insurance pricing, portfolio management, and sensor optimization.THE BIGGER PICTURE It can be advantageous to combine multiple predictive models for power or robustness, but it is recognized that realizing these potential gains cannot be guaranteed, especially when the input models cannot be appropriately weighted a priori or the resulting fusion models cannot be cross-validated. This, and a series of mathematically related problems in different guises, fundamentally limits the ability to optimally use all available models to improve classification across essentially all domains in which more than one potentially useful model exists. We show that any fusion's outcome is fully predictable/explicable given characterization of the models to be fused. The ''mechanism'' described acts at the level of ranks, not scores, which extends our findings to all distributions and, functionally, to any domain of interest. We are elucidating the underlying math, following the framework's implications for data science, and using this approach on real-world problems.Concept: Basic principles of a new data science output observed and reported