This technical note from the US Agency for International Development (USAID) provides an overview to using a mixed-options approach for evaluation and outlines some of the important considerations that must be taken into account when using the MM approach.
Frequently, evaluation statements of work specify that a mix of options be used to answer evaluation questions. This guide includes the rationale for using a mixed-option evaluation design as well as guidance for selecting among options (with an example from an evaluation of a training program) and examples of techniques for analyzing data collected with several different options (including parallel analysis).
Vondal, P (2010) Conducting Mixed-Option Evaluations Performance Monitoring & Evaluation TIPS Washington DC, USAID Center for Development Information and Development Evaluation. Retrieved from http://transition.usaid.gov/policy/evalweb/documents/TIPS-ConductingMixedMethodEvaluations.pdf