nested case-control study 

EVIDENCE-BASED MEDICINE 

A type of case-control study in which subsets of controls from the cohort are compared to incident cases, with each subset selected from a case’s matched risk set.

By matching factors such as age and selecting controls from relevant risk sets, the NCC design is more efficient than a case-cohort design with the same number of selected controls.

NCC studies are used when the exposure of interest is difficult or expensive to obtain and when the actual outcome rarely occurs. 

References NEJM 1997; 337; 69oa

http://en.wikipedia.org/wiki/Nested_case-control_study