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ICEBOH. Split-mouth studies and systematic reviews. Ian Needleman 1 & Helen Worthington 2 1 Unit of Periodontology UCL Eastman Dental Institute International Centre for Evidence-Based Oral Health, London UK 2 School of Dentistry, University of Manchester, UK.

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Split mouth studies and systematic reviews

ICEBOH

Split-mouth studies and systematic reviews

Ian Needleman1 & Helen Worthington2

1Unit of Periodontology

UCL Eastman Dental Institute

International Centre for Evidence-Based Oral Health, London UK

2School of Dentistry, University of Manchester, UK


Split mouth studies and systematic reviews1
Split-mouth studies and systematic reviews

  • What is the issue?

  • Why include them?

  • How to include them

  • Examples


Split mouth studies and systematic reviews the analysis issue
Split-mouth studies and systematic reviews – the analysis issue

  • As we know, the analysis of split-mouth and parallel group studies is not the same.

  • As a result, if a meta-analysis includes both types of trials without considering the differences, the result might be unreliable

  • The confidence interval will be incorrect, possibly leading to;

    • An inappropriate conclusion on clinical importance (and statistical significance)

    • Distortion of impact of clinical heterogeneity


Why include split mouth studies in systematic reviews
Why include split-mouth studies in systematic reviews? issue

  • Because of the totality of the evidence

  • Possible advantages of split-mouth trials over parallel group;

    • Each participant acts as own control

    • Therefore, fewer participants are required to obtain same study power as parallel group

    • Every participant receives each intervention, therefore good for determining preferences


How to include split mouth studies in systematic reviews
How to include split-mouth studies in systematic reviews issue

Designing the systematic review:

  • Is split-mouth an appropriate design to answer this question?

  • Are carry-over effects a risk?


How to include split mouth studies in systematic reviews1
How to include split-mouth studies in systematic reviews issue

Conducting the systematic review:

  • Fundamental question: Is meta-analysis justified in principle?

  • Are the trials similar enough in chief characteristics:

    • Types of populations

    • Types of interventions

    • Types of outcomes


How to include split mouth studies in systematic reviews2
How to include split-mouth studies in systematic reviews issue

Possible ways:

  • Narrative (qualitative) summary in evidence tables only.

    • Advantage: split-mouth studies contribute to totality of evidence and analytic issues (may be) avoided

    • Disadvantage: Do not contribute to summary estimate or to investigation of heterogeneity

  • Analyse as if parallel group

    • Not recommended due to potentially unreliable meta-analysis summary estimate


How to include split mouth studies in systematic reviews3
How to include split-mouth studies in systematic reviews issue

Possible ways:

  • Meta-analyse those split-mouth trials with adequate data separately from parallel group trials and ignore those without such data

    • Advantage: More information

    • Disadvantage: Selection bias

  • Incorporate data from first intervention side if reported separately

    • Advantage: More information

    • Disadvantage: May be biased sample where trialists identified carry-over effects


How to include split mouth studies in systematic reviews4
How to include split-mouth studies in systematic reviews issue

Possible ways:

  • Approximate a paired analysis by inputing a measure describing the similarity of outcomes within each participant.

    • Advantages: Makes use of all trials

    • Disadvantages:

      • May make assumptions about data (that can be tested)

      • May need statistical support


Approximating a paired analysis you will need one of the following
Approximating a paired analysis - you will need one of the following

  • Individual patient data in publication or from contact with trialist

  • Mean and SD/SE of patient specific differences between intervention A and B measurement


Approximating a paired analysis you will need one of the following1
Approximating a paired analysis - you will need one of the following

  • Mean difference (or difference between means) and one of:

    • t-statistic (paired t-test)

    • P-value from paired t-test

    • Confidence interval from paired analysis

  • Graph of measurement of intervention A and B from which matched individual data values can be extracted.


  • Approximating a paired analysis
    Approximating a paired analysis following

    • Step one. Calculate the correlation coefficient (r) for each study.

      • The correlation coefficient describes how similar the measurement of intervention A and B were within a participant

      • Assumes that mean and SD for each intervention treatment side/period would be the same

      • If r is inconsistent between studies, then caution on proceeding further.

      • If r cannot be calculated for a trial, use representative value from other trials as the trials should be very similar for a particular intervention in a systematic review


    Approximating a paired analysis1
    Approximating a paired analysis following

    • Step two. Calculate SE of the mean difference between the interventions

    • Step three. Enter the data into meta-analysis software. Generic inverse variance method of Cochrane Software - RevMan - particularly flexible (free download).

    • Step four. Conduct sensitivity analyses employing different values for r to investigate robustness of estimates especially for studies were r could not be calculated


    How to include split mouth studies in systematic reviews5
    How to include split-mouth studies in systematic reviews following

    Further issues:

    • Generally advisable to meta-analyse split-mouth and parallel group trials separately as sub-groups to investigate systematic differences

    • Authors of systematic reviews: please state explicitly how data from split-mouth studies has been managed


    How to include split mouth studies in systematic reviews6
    How to include split-mouth studies in systematic reviews following

    Example of meta-analysis of split mouth and parallel group trials.


    Needleman, followingWorthington, Giedrys-Leeper, & Tucker 2006,

    Cochrane Database of Systematic Reviews


    Split mouth studies and systematic reviews2
    Split-mouth studies and systematic reviews following

    • Conclusions

      • Split-mouth studies should be included in systematic reviews when appropriate

      • It is possible to combine split-mouth and parallel group studies in meta-analysis

      • Our observation within the Cochrane Oral Health Group is that differences in effect sizes exist between split-mouth and parallel group studies of the same intervention


    Split mouth studies and systematic reviews3

    ICEBOH following

    Split-mouth studies and systematic reviews

    • Resources

      • Cochrane Handbook and RevMan: www.cochrane.org

      • Elbourne et al. 2002. Int J Epidem, 31: 140-149

      • Follman et al. 1992. J Clin Epidem, 45: 769-773

    • Contact: [email protected]


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