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Coding data

Trudy Bekkering

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What data should you collect?

• comprehensive information about each study • population and setting• interventions and integrity of delivery

Extract factors and variations that may have impact on results of studyExtract information to decide whether results can be applied in their context.

• methods and potential sources of bias• outcomes, results, authors’ conclusions• citation, author contact details• sources of funding, etc.

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What data should you collect?

• comprehensive information about each study • population and setting• interventions and integrity of delivery• methods and potential sources of bias• outcomes, results, authors’ conclusions• citation, author contact details• sources of funding, etc.

• information required for:• references• description of included studies• risk of bias assessment• analyses

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Collecting outcome data

• focus on those outcomes specified in your protocol• be open to unexpected findings, e.g. adverse effects

• measures used• definition (e.g. diagnostic criteria, threshold)• timing• unit of measurement • for scales – upper and lower limits, direction of benefit, modifications,

validation, minimally important difference• numerical results

• may be in many formats - conversion may be required• collect whatever is available• no. participants for each outcome & time point

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Data in many formatsOutcome Reported as TrialsVolume transfused (mls) Mean and SEM

Mean and SDMean and something in bracketsMedian and something in bracketsTwo unlabelled numbers e.g. x(y)Bar chart showing mean per person per day

421111

Units transfused Mean and SEMMean onlyTotal in each group

111

Volume adjusted for patient mass (mls/kg)

Mean and SD 1

Patients who had a transfusion Number of patients 3

Not measured Not reported 1

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Data collection forms

• organise all the information you need• reminds you what to collect• records what was not reported in the study• records the decisions you make about each study• source document for data entry into your review

• must be tailored to your review• adapt from a good example

• paper or electronic – your choice

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Hints and tips

• plan what you need to collect – not too much or too little• consider including:

• name of author completing the form• Study ID (and Record ID if multiple reports of a study)• plenty of space for notes (at beginning and throughout)• source of each piece of information (e.g. page no.)• tick boxes or coded options to save time• ‘not reported’ and ‘unclear’ options• format to match RevMan data entry

• provide instructions for all authors

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What Paper coding form

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Excel coding form

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Minimising bias in data collection• two authors should independently collect data

• reduces error• ensures agreement on subjective judgments and interpretations

• resolving disagreements• can usually be resolved by discussion• if not, refer to a third author

• pilot data collection process• include each person assisting• ensure criteria are consistently applied• may need to revise form or instructions

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Managing data

• can enter data directly from form to RevMan• may need to consider intermediate steps• e.g. Excel spreadsheet• group studies by comparison & outcomes measured• calculations to convert into required statistics

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Data needed for meta-analyses• Dichotomous data

Pain No pain

Intervention 20 80

Control 40 60

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Data needed for meta-analyses• Continuous data

N Mean SD

Intervention 50 2.3 1.03

Control 60 2.2 1.2

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Available data for review

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Take home message

• think carefully about the data you wish you collect• design and pilot a data collection form• should be done independently by two authors to

minimise error and bias

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References

• Higgins JPT, Deeks JJ (editors). Chapter 7: Selecting studies and collecting data. In: Higgins JPT, Green S (editors), Cochrane Handbook for Systematic Reviews of Interventions Version 5.0.1 (updated September 2008). The Cochrane Collaboration, 2008. Available from www.cochrane-handbook.org

Acknowledgements• Compiled by Miranda Cumpston• Based on materials by the Australasian Cochrane Centre• Approved by the Cochrane Methods Board