Incomplete system requirements specifications can lead to severe budget, quality, and project risks. Selecting methods during process improvement that lead to high completeness provides an opportunity to change this situation. This thesis presents CAB-IM an approach for the systematic selection of requirements specification methods with regards to completeness. CAB-IM provides (1) a conceptual model of completeness, (2) an assessment, and (3) an evaluation procedure that in conjunction allow to compare the effect of different requirements engineering methods on the completeness of the produced specification.