stratification in nonparametric roc studies in many clinical experiments it is clear that external factors can affect the performance of diagnostic tests as these factors influence the distributions of separator variables the authors investigate how stratification of cases andor controls can be used in nonparametric received operating characteristic roc studies and develop statistical methods using stratification the method can be used to simultaneously assess the ability of a diagnostic marker against several types of controls or cases this allows a new method for the assessment of diagnostic tests effective fragment potential study of the interaction of dna bases hydrogenbonded and stacked structures of adeninethymine and guaninecytosine nucleotide base pairs along with their methylated analogues are examined with the ab inito based general effective fragment potential efp2 method a comparison of coupled cluster with single double and perturbative triple ccsdt energies is presented along with an efp2 energy decomposition to illustrate the components of the interaction energy