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Carrie Bennette on behalf of Andrew Vickers

How do we know whether a marker or model is any good? A discussion of some simple decision analytic methods. Carrie Bennette on behalf of Andrew Vickers Pharmaceutical Outcomes Research and Policy Program (PORPP) University of Washington. Overview of talk.

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Carrie Bennette on behalf of Andrew Vickers

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  1. How do we know whether a marker or model is any good?A discussion of some simple decision analytic methods Carrie Bennette on behalf of Andrew Vickers Pharmaceutical Outcomes Research and Policy Program (PORPP) University of Washington

  2. Overview of talk • Marker research in cancer: state of the science • Traditional statistical methods for evaluating predictions • Decision analytic approaches

  3. Overview of talk • Marker research in cancer: state of the science • Traditional statistical methods for evaluating predictions • Decision analytic approaches

  4. A combination of common and minor variations in five regions of DNA can help predict a man’s risk of getting prostate cancer, researchers reported Wednesday. A company formed by researchers at Wake Forest University School of Medicine is expected to make the test available in a few months …. It should cost less than $300. This is, some medical experts say, a first taste of what is expected to be a revolution in medical prognostication

  5. SNP panel • Predictive accuracy of SNP panel (as calculated by AV): 0.57 • Predictive accuracy of single PSA in middle age: 0.75 • Doesn’t add to standard predictors (Nam et al.)

  6. Systematic review of molecular markers in cancer • 129 papers published in 2005 and 2006 eligible for analysis • More markers than papers • 97% included inference statistics • 36% included marker in a multivariable model • 11% measured predictive accuracy • 0 used decision analytic techniques

  7. Overview of talk • Marker research in cancer: state of the science • Traditional statistical methods for evaluating predictions • Decision analytic approaches

  8. Example: Binary test for cancer on biopsy • Patients with high PSA are referred to biopsy • But most patients with high PSA don’t have prostate cancer • Could a second marker help? • Study of biopsy cohort: 26% had cancer • Assess presence of two markers

  9. Traditional biostatistical metrics

  10. Which test is best? • Sensitivity / specificity insufficient to determine which test should be used: • “Depends on whether sensitivity or specificity is more important”

  11. Conclusion about traditional metrics • Traditional biostatistical techniques for evaluating models, markers and tests do not incorporate clinical consequences • Accordingly, they cannot inform clinical practice

  12. Overview of talk • Marker research in cancer: state of the science • Traditional statistical methods for evaluating predictions • Decision analytic approaches

  13. Threshold probability • Predicted probability of disease is p̂ • Define a threshold probability of disease as pt • Patient accepts treatment if p̂ ≥ pt • pt describes how patients values relative harm of false positive and false negative

  14. Decision theory “I would biopsy a man if his risk of prostate cancer was 20% or more, that is, I would conduct no more than 5 biopsies to find one cancer. I consider the harms associated with delaying the diagnosis of prostate cancer to be four times worse than the harms, risks and inconvenience of biopsy.”

  15. Worked example at pt of 20%

  16. Net benefit has simple clinical interpretation • Net benefit of 0.126 at pt of 20% • Using the model is the equivalent of a strategy that led to 126 patients per 1000 with cancer being biopsied with no unnecessary biopsies

  17. Net benefit has simple clinical interpretation • Difference between model and treat all at pt of 20%. • 0.051 • Divide by weighting 0.051/ 0.25 = 0.204 • 204 fewer false positives per 1000 patients for equal number of true positives • E.g. 204 fewer patients undergoing biopsy without missing any cancers

  18. Decision curve analysis 1. Select a pt 2. Positive test defined as 3. Calculate “Clinical Net Benefit” as: 4. Vary ptover an appropriate range Vickers & Elkin Med Decis Making 2006;26:565–574

  19. Decision analysis All markers Free, Total PSA PSA Biopsy all Biopsy none Vickers JCO 2009

  20. Gallina vs. Partin AUC 0.81 AUC 0.78 P=0.02

  21. Decision curve analysis

  22. Conclusion • Huge number of markers proposed • Evidence base is very weak for most • Traditional biostatistical methods do not assess clinical value of a marker • Simple decision analytic methods can distinguish potentially useful from useless models and markers

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