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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?
  • 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

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

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

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

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

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
  • 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
  • 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

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

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

slide16

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

Cochrane Database of Systematic Reviews

split mouth studies and systematic reviews2
Split-mouth studies and systematic reviews
  • 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

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: i.needleman@eastman.ucl.ac.uk
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