Different understanding of the nature of validity leads to diverse validity theory and validation practice The positivistic paradigm focuses on certainty and generalization The naturalistic paradigm emphasizes the degree to which the evaluator and evaluation audience can agree on, and feel confident about, an interpretation of the evaluation findings The challenges of validity research are: reliability and validity, the unitary and the divisible, and validity and validation, etc The key notion is that although it is clear that important differences remain between positivistic and naturalistic perspectives on validity, validity is establishing what counts as evidence We have seen that the criteria for doing this can differ depending on the research perspective being used