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Measures of Association

Measures of Association. Professor Mobeen Iqbal Shifa College of Medicine. How large was the treatment effect? Measures of Association. Risk Relative Risk Relative risk reduction Absolute risk reduction Numbers needed to treat. Risk or Event rate.

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Measures of Association

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  1. Measures of Association Professor Mobeen Iqbal Shifa College of Medicine

  2. How large was the treatment effect?Measures of Association • Risk • Relative Risk • Relative risk reduction • Absolute risk reduction • Numbers needed to treat

  3. Risk or Event rate the number of people experiencing an event as a proportion of the number of people in the population • Experimental Event Rate (EER) • Control Event Rate (CER)

  4. Relative Risk RR is calculated the rate of exposed or treated divided by those not exposed or treated RR= EER/CER

  5. Relative Risk (RR) Control event rate (CER)=a/a+b= 719 / (719+5584) =11% Experimental event rate (EER)=c/c+d= 582/ (582+5584)= 9% RR = EER/CER =9/ 11 = 80% Exp ODDS=a/b=582/5677 Con ODDS=c/d=719/5584 Odds Ratio=a/b c/d

  6. Relative Risk • RR = 1 no effect (no difference) • RR = < 1 protective • RR = > 1 increase risk

  7. Relative Risk Reduction (RRR) • the difference in event rates between 2 groups, expressed as a proportion of the event rate in the untreated group; usually constant across populations with different risks Control event rate (CER) = 719 / (719+5584) =11% Experimental event rate (EER) = 582/ (582+5584) = 9% RR = EER/CER =9/ 11 = 0.8 RRR = 1-RR (CER-EER/CER=1-0.8 = 20 %

  8. Absolute risk reduction (ARR) the arithmetic difference between 2 event rates; varies with the underlying risk of an event in the individual patient Control event rate (CER) =11% Experimental event rate (EER) = 9% RR = EER/CER = 80% RRR = 1-RR (CER-EER/CER) = 20% ARR = CER – EER = 11% - 9% = 2%

  9. Number Needed to Treat (NNT) • the number of patients who would have to receive the treatment for 1 of them to benefit; • calculated as 100 divided by the ARR expressed as a percentage (=1/ARR) RR = EER/CER = 80% RRR = 1-RR (CER-EER/CER) = 20% ARR = CER – EER = 2% NNT = 1/ARR =1 / 2 x100 =50

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