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Table 2 Questions for assessing external validity (EV)

From: Checklist for the qualitative evaluation of clinical studies with particular focus on external validity and model validity







Study population – assessment of selection bias (related to EV)

• To what extent do the inclusion and exclusion criteria (where relevant, other selection criteria) define the "everyday or target population" of the intervention?


• Does the applied diagnostic procedure reflect everyday conditions and the everyday possibilities (access, necessity) respectively?


• Are the diagnostic procedures and evaluations performed by persons with similar qualification and experience as in everyday practice?


• Does the study population reflect the everyday population in terms of:


    Severity of the illness


    Duration of illness


    Intra-individual variability






    Further socio-demographic characteristics


    Therapy preferences and expectations


    Symptoms of side effects of the interventions


    Accompanying illnesses


    Accompanying medication


    Further prognostic or therapy relevant parameters?


• Has the structural similarity between the study and the everyday population or target population been tested?

Intervention und control – assessment of performance bias (related to EV)

• Does the preparation (medication, other medicinal products, other kind of interventions) reflect the usual treatment?


• In case of medication, does the dosage reflect the usual treatment? (Is dose modification possible?)


• Does the type of administration reflect the usual treatment?


• Does the intervention duration reflect the usual treatment duration?


• Are the permitted accompanying treatments the usual accompanying treatments?


• Does the study situation reflect the common treatment situation?


• Are the interventions carried out by therapists with similar qualifications and experience as in everyday practice?

Outcome measurements, results and evaluation – assessment of detection and attrition bias (related to EV)

• Are the chosen outcomes practice and patient relevant? (E.g. no surrogate parameter, are individual therapy goals defined?)


• Were the following important outcomes considered: quality of life, subjective health, patient's general evaluations, compliance, reasons for dropout, use of accompanying treatments, rebound effect following termination of treatment (or, for example, symptom deceit)?


• Are the test procedures used in usual practice?


• Are the tests and evaluations performed by persons with similar qualifications and experience as in every day practice?


• Are the differences clinically relevant?


• Were sufficient data collected to cover the intra-individual variability?


• Do the test conditions reflect the everyday practice?


• Does the dropout rate reflect everyday experience? Are the reasons for dropout registered (e.g. adverse effects, insufficient effect), so that the significance for the everyday effectiveness can be assessed?


• Is clinical relevance considered in the conclusion?

Study design and Setting (related to EV)

• Is the research question clinically relevant?


• Does the study design ensure a high EV?


• Does the study setting reflect the everyday conditions?


• Are the investigators the regular contact persons (e.g. GP or relevant clinic doctor, or are they at least comparable in terms of training, status, experience, preferences; does the number of contact people reflect the usual setting)?


• Does the doctor/therapist-patient relationship reflect the everyday conditions (e.g. frequency of contact, constant contact person)?

  1. + Matches completely/is completely fulfilled
  2. (+) Matches incompletely but sufficiently/is only partly but sufficiently fulfilled
  3. - Does not match or matches insufficiently/is insufficiently fulfilled
  4. c.b.e. Can not be evaluated