| New policy with a clearly defined inception date or administrative jurisdiction |
Gun control laws are implemented at state level within the US, so the effect of introducing or withdrawing a law in one state can be evaluated using other states as controls29
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| Policy that applies to some members of a population but not others |
Social security policies often define eligibility for payment in terms of age or income, allowing comparisons between individuals with ages or incomes just above or just below the eligibility cut-off31
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| Phased implementation of a policy across a population |
Universal Credit, a new system of social security benefits and tax credits in the UK, was rolled out area by area, allowing comparisons of recipients of benefits under the old system and the new system using data from surveys running throughout the rollout period32
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| Policies using randomisation as an assignment mechanism, generating otherwise similar exposed and unexposed individuals or groups |
Election laws in two Indian states required a minimum number of council seats be reserved for women in a randomly selected proportion of villages, allowing comparisons of decision making in villages with larger or smaller numbers of women involved34
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| Flaws or shortcomings in policy implementation |
Database errors led to people in some areas being randomly excluded from England’s test and trace programme for covid-19 until the error was noticed. This enabled trends in infection rates to be compared in areas with and without tracing in operation14
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