General Elimination Methodology (GEM) is a theory-driven qualitative evaluation method that improves our understanding of cause and effect relationships by systematically identifying and then ruling out causal explanations for an outcome of interest (Scriven 2008; White and Phillips 2012).

Often used as a post-hoc evaluation method, it supports a better understanding of complexity.

Also called the Modus Operandi Approach, GEM is compared to detective work, where a list of suspects are ruled out based on the presence or absence of motive, means and opportunity. In this respect, it describes an approach to thinking about causation that we all use, subconsciously, on a regular basis.

GEM has some similarities with process tracing, which could be seen as a more complex version of GEM and, therefore, GEM could be a useful introduction to some of the more complex small n methodologies.

What is involved?

GEM involves three primary steps (Scriven 2008; White and Phillips 2012):

Step 1: Establish a ‘List of Possible Causes’

First, the evaluators identify all the possible causes for the impact of interest. Secondly, they identify the necessary conditions for each possible cause and assess whether these conditions are present. This work can be based on secondary data analysis, such as a review of reports, articles, websites and other sources generally used to build a Theory of Change. The evaluators then need to identify rival explanations for the outcome of interest. This is generally achieved by engaging stakeholders in interviews or workshops.

Step 2: List the modus operandi for each cause

A modus operandi (MO) is a sequence of events or set of conditions that need to occur/be present for the cause to be effective. In investigative terms, detectives (i.e. the evaluators) set a list of means, motives and opportunity which are considered for each suspect (i.e. each cause). The list of MO helps evaluators decide whether certain conditions should be included or rejected.

Step 3: Assess each case against the evidence available

For each possible cause, the evaluator will consider the presence or absence of the factors identified in the modus operandi, and only keep those whose modus operandi is completely present.
Download a General Elimination Theory case study here
Download a longer briefing of the General Elimination Theory here

Useful resources

There is very little published material on General Elimination Methodology. The key paper in which GEM was first described in the terms set out here was:

Scriven, M. (2008) ‘A Summative Evaluation of RCT Methodology & An Alternative Approach to Causal Research’, Journal of Multi-Disciplinary Evaluation, 5(9) 11–24. Available here.

For an interesting discussion about causation between Scriven (the originator of GEM) and Cook (a proponent of ‘traditional’ models of counterfactual impact evaluation), see:

Cook, T.D., Scriven, M., Coryn, C. L. S., Evergreen, S. D. H (2010) ‘Contemporary Thinking About Causation in Evaluation: A Dialogue with Tom Cook and Michael Scriven’, American Journal of Evaluation, 31(1) 105–117. doi:10.1177/1098214009354918