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Different types of trial design. and implications for reporting Alison Wearden. Uncontrolled trials. Test feasibility and acceptability of an intervention, and whether there are adverse effects May allow preliminary examination of mechanisms of change

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Different types of trial design l.jpg

Different types of trial design

and implications for reporting

Alison Wearden

Uncontrolled trials l.jpg

Uncontrolled trials

  • Test feasibility and acceptability of an intervention, and whether there are adverse effects

  • May allow preliminary examination of mechanisms of change

  • N-of-1 designs, randomized schedule, can be used to test theory (cause and effect)

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From D. Wang & A Bakhai (Eds) Clinical Trials. A Practical Guide

to Design, Analysis and Reporting. 2006, Remedica: London, pp 57

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The hegemony of pharmacological trials

  • Blinded randomised controlled trials considered gold standard for testing drugs

  • Heavily influenced guidelines for RCTs

  • Are drug-RCT issues always appropriate for complex behavioural interventions?

  • Can attempts to eliminate bias distort implementation and assessment of intervention?

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What factors do you need to take into account choosing a trial type and design?

What is your research question?

Are there ethical constraints?

Where might you want to publish your trial?



Parallel arms/








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Explanatoryvs pragmaticEfficacyvs effectiveness

Explanatory, efficacy

Does this intervention work in people who receive it under carefully controlled conditions? How does it work?

Pragmatic, effectiveness

What would be the outcome if this intervention were implemented in usual clinical practice?

  • Broad, inclusive sample

  • More flexible

  • implementation

  • Qualitative studies, e.g. to

  • determine why people

  • dropped out

  • Tightly defined sample

  • Fixed protocol

  • Compliance measured

  • Process measures

See Zwarenstein et al., 2008, BMJ, 337, 1223-6

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Intention to treat analysis

  • Preserves the benefits of randomization, ie minimizes bias. Withdrawals may be more common in one arm of trial

  • Ecological validity (people do drop out)

  • It gives a pragmatic estimate of the effectiveness of a treatment, rather than just a report of the efficacy of the treatment

  • It requires a method for dealing with missing data

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Equivalence trials

  • Instead of having a “no treatment” control, equivalence trials test whether a new intervention is as good as (or not worse than) an established treatment with proven efficacy

  • New intervention may have some advantage (convenience, cost) over established, but is it at least as efficacious?

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Randomisation issues

  • Unit of randomisation (individual, group)

  • Simple or block design (to even up group numbers)

  • Stratification or minimisation to achieve groups balanced on key baseline characteristics

  • Patient preference designs

    • Zelen

    • Wennberg

      • Rucker

    • Comprehensive cohort

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Treatment A

(May be

analysed as






Receives A


Treatment B


Receives B

Preference designs

  • Patient preference may be an issue

    • refusal to participate

    • reduced compliance with non-preferred arm

  • Consent may be taken AFTER randomization, e.g. Zelen design:

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Some trial designs



Randomised to

A or control


Simple, parallel design

Is A better than control?

Factorial design

Are effects of A and B

additive or interactive?

A & B each controlled

Cross-over design

(Unlikely – useful with small samples)


Randomised to







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Analysis issues

  • Pre-specification of primary outcomes if there are multiple measures

  • Analysis plan pre-specified and published

  • Intention to treat vs per protocol analysis

  • How are missing data dealt with?

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Where do you want to publish?

  • Which audience do you want to reach?

  • Does impact factor matter?

  • Uniform requirements for “prestigious” medical journals

    • Trial must be registered

    • Protocol must be published

    • CONSORT guidelines must be adhered to

  • Important issue of publication bias

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Special considerations for reporting different types of trials

  • Standard CONSORT guidance is at


    • designated primary outcomes

    • sequence generation

    • allocation concealment ….

  • CONSORT guidelines have been elaborated for trials with complex interventions (cf. drug trials) and non-simple designs

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Reporting of psychological trials should include

  • Components of the intervention(s), how they were individualised (if applicable), how they were standardized

  • How therapist fidelity to treatment assessed

  • How experimental intervention and comparator(s) were implemented

  • Description of therapists (specialism, experience etc) and setting

Boutron et al., 2008, Ann Int Med, 148, 295-309

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Reporting pragmatic trials

  • Explanatory vs pragmatic “attitude”

  • Sample description and eligibility criteria – is sample typical? Is setting typical?

  • CONSORT diagram should explain reasons for non-participation if known

  • Particular attention to clinical interest of findings

  • Description of key aspects of setting which affected findings

Zwarenstein et al., 2008, BMJ, 337, 1223-6

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Issues with equivalence, or non-inferiority, trials

  • People may be carrying out equivalence trials without realising it.

  • Analysis with respect to a pre-stated margin of non-inferiority (smallest clinically interesting difference)

  • ITT analysis may increase risk of type 1 error

  • Choice of outcomes important

Piaggio et al., 2006, JAMA,295,1152-1160

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Reporting equivalence trials

  • Need to reference established efficacy of “standard” treatment

  • Hypotheses should be framed in terms of non-inferiority

  • “Margins of equivalence” should be reported

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