Strategy Survival Guide

groups at random. In other words, the two groups are statistically equivalent, the only systematic difference
between them being that the programme group has been exposed to the policy being investigated.
Evaluators can randomly assign individuals, or other units such as institutions (for example hospitals or
schools), or geographical areas (for example Wards, or Local Authority Districts).
At present, this approach, while commonplace in clinical trials, is less often used to evaluate social
programmes in the UK, although there are examples. It is, however, widely used in North America to
investigate the impact of various interventions from changes in taxation, welfare reform programmes,
initiatives in education and criminal rehabilitation.
Strengths of random assignment
• If implemented correctly, it guarantees that the experimental and control groups will be identical.
Thus it eliminates the influence of extraneous factors by ensuring that the only differences between
the two groups arise by chance.
• Easy to interpret.
Weaknesses of random assignment
• Two groups are unlikely to be identical apart from some policy intervention .
• Only provides a measure of average impact.
• Can be complicated to implement correctly- two administrative systems are required.
• Can create political problems by denying services to controls.
• Risk of contamination if those in the control group are not prevented from participating in the pilot
programme.
Many of these practical problems can be avoided if whole areas are divided into intervention and control
groups, but for practical reasons this is usually difficult to do.
References
Cost Benefit Analysis, Boardman, Greenberg, Vining and Weimer (2001)
Research Methods for Policy Evaluation, Department for Work and Pensions, Research Working paper No 2.
(Chapter 4 gives an excellent description of counterfactual analysis and the different methods available.)
Strategy Survival Guide – Strategy Skills
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