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Definitions (38)

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attrition


Attrition, also known as dropout, occurs when participants fail to comply with the study requirements or leave a study after they have been assigned to an experimental group. It can lead to a biased estimate of the effect size because those that drop-out are likely to be different from those that stay in. For example, less motivated officers or tea [..]
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bias


A study is biased if its impact estimate varies from the real impact. This variation can be linked to weakness in the implementation or design of the evaluation.
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blinding


Blinding is where information about the assignment of participants to their experimental group (e.g. control or treatment) is concealed from the evaluator, the participants, or other people involved in the study until it is complete.
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confidence intervals


All of the effect sizes produced by impact evaluations are estimates. Typically, confidence intervals provide the range of values that has a 95% probability of including the real effect size. The width of the confidence interval indicates the confidence we can place in a finding: the wider the interval, the less confidence we can have.
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control group


Sometimes called a "comparison group", this group does not receive the intervention being evaluated and allows the evaluator to estimate what would have happened if the treatment group had not received the intervention. The control group should be as similar to the treatment group as possible before the intervention is applied. Th [..]
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counterfactual


The outcome for the treatment group if it had not received the intervention is called the counterfactual. If a control group is constructed correctly, it can be used to estimate the counterfactual.
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effectiveness trial


Effectiveness trials aim to test the intervention when implemented 'at scale' under realistic conditions in a large number of forces, boroughs or divisions. A quantitative impact evaluation is used to assess the impact on crime and a process evaluation is used to identify the challenges for delivery at scale. The cost of the intervention [..]
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effect size


The effect size is an estimate of the size and direction of a change caused by an intervention. The value of an effect size is that it quantifies the effectiveness of a particular intervention, relative to a comparison group – essentially a measure of the extent
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efficacy trials


Efficacy trials test whether an intervention can work when implemented under controlled conditions and on a small organisational scale. Experimental design
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external validity


Describes the extent to which the results of an evaluation apply to another context. For example, a study which finds that an intervention is effective in an inner city division may have poor external validity in a rural division, because the areas may have different demographic profiles and crime rates.
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