Define and describe what is being evaluated

It is important to define and describe what is being evaluated. This enables a shared understanding and allows comparison with other evaluations conducted elsewhere.
  • Describing the intervention

    The description should include:

    • The location(s) of the intervention (named station/s route etc);
    • The intervention(s) itself – a written description and also category of intervention (e.g. awareness/education; community engagement; environmental; physical; monitoring & surveillance; staffing and patrols and fines);
    • Intervention time scales;
    • Type of trespass being targeted;
    • Type of trespasser being targeted;
    • Time periods being targeted.

    These points may be drawn from previous trespass risk assessment.

    An example description: The installation of ‘witches’ hats’ and gates to all platforms ends at all stations in region x between January and December 2020, aiming to reduce the frequency of all forms of trespass by all types of trespassers from platform ends, with a reduction being measurable by March 2021.

  • Logic model

    A logic model describes how an intervention is expected to have an impact.  The purpose of a logic model is to provide a rational basis for identifying and defining evaluation measures and for specifying the expected link between the intervention’s inputs/outputs and its outcomes and impacts. This helps to ensure a valid set of evaluation measures are specified. A logic model usually includes:

    Inputs: What will inputted to make the intervention possible? Two examples being:

    1. funding for witches’ hats at 50 stations;
    2. training of 25 community safety staff).

    Outputs: What will the intervention deliver? Two examples being: 

    1. the installation of witches’ hats by Network Rail at 50 stations; 
    2. 250 community safety events each year).

    Outcomes: What will the intervention achieve? Two examples being: 

    1. a reduction in the ability and motivation of people to trespass via end of platforms; 
    2. increase in awareness of trespass hazards amongst local community).

    Impacts: what will be the impact of the intervention? Two examples being: 

    1. 50% reduction in frequency of end of platform trespass within three months of installation;
    2. 10% reduction in all forms of trespass).

    Impact logic model diagram

    A logic model helps to identify the inputs, outputs and outcomes that a process evaluation, as discussed below, should assess. 

    This should clearly note the expected scale of impact, the impact timescale, the type of trespass and type of trespasser(s). 

    It may not be possible to provide a numerical prediction. If so, a qualitative prediction may be given, such as “a small reduction in the number of trespass events”. 

    To avoid subjectivity and inconsistency in the description of an impact, a standard set of terms and magnitude categories can be used.  Figure 3 provides a way of categorising and describing the magnitude of impact. The categories cover the reported magnitude of impact for available trespass prevention methods. The use of an impact “band” such as 40% to 59% is consistent with the level to which predictions can be made. 

    Describing the predicted magnitude of effect diagram
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Get in touch with our Senior Safety Intelligence Analyst for further information.
Siona Vass
Tel: 020 3142 5485
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