1. Which Treatment ? For a patient, 64-year-old male For a patient, 64-year-old male No HT, DM, Heart failure, LV dysfunction No HT, DM, Heart failure, LV dysfunction Having non-rheumatic atrial fibrillation Having non-rheumatic atrial fibrillation Treatment ‘A’ reduces annual stroke risk by 40% Treatment ‘A’ reduces annual stroke risk by 40% Treatment ‘B’ requires 250 patients to be treated for one year to prevent one stroke Treatment ‘B’ requires 250 patients to be treated for one year to prevent one stroke Treatment ‘C’ reduces annual stroke risk from 1.0% to 0.6% Treatment ‘C’ reduces annual stroke risk from 1.0% to 0.6%

2. Newspaper headlines: Economic reforms succeed Economic reforms led to decline in unemployment from 20% to 15%, thus reducing it by 25% Economic reforms led to decline in unemployment from 20% to 15%, thus reducing it by 25%

3. Newspaper headlines: Economic reforms succeed Economic reforms led to decline in unemployment from 20% to 15%, thus reducing it by 25%. Economic reforms led to decline in unemployment from 20% to 15%, thus reducing it by 25%. How? Difference is only 5%. How? Difference is only 5%. If 20% = 100% If 20% = 100% Then 15% = 75% Then 15% = 75% And 5% = 25% And 5% = 25%

4. Different ways of expressing effects Economic reforms led to decline in unemployment rate from 20% to 15%, thus reducing it by 25% Economic reforms led to decline in unemployment rate from 20% to 15%, thus reducing it by 25% 20% - 15% = 5% 20% - 15% = 5% If 20% = 100% If 20% = 100% Then 15% = 75% Then 15% = 75% And 5% = 25% And 5% = 25% Baseline risk Relative risk Relative risk reduction Absolute risk reduction

5. In an Intervention trial Intervention led to decline in risk (incidence) of mortality from 20% (in control group) to 15% (in intervention group), thus reducing it by 25%. Intervention led to decline in risk (incidence) of mortality from 20% (in control group) to 15% (in intervention group), thus reducing it by 25%. Risk Difference is the simple difference between the two risks = 5%. Risk Difference is the simple difference between the two risks = 5%. If 20% is taken as 100%, then If 20% is taken as 100%, then 15=75% = RR (15%/20%=0.75=75%) 15=75% = RR (15%/20%=0.75=75%) And 5% = 25% i.e. RRR (100-75 i.e. 100- RR (%) or in decimals, 1 - RR And 5% = 25% i.e. RRR (100-75 i.e. 100- RR (%) or in decimals, 1 - RR

6. What does risk difference of 5% mean? What does risk difference of 5% mean? 5 per cent = 5 per 100, 5 per cent = 5 per 100, 5 less death per 100 need to be treated with new t/t 5 less death per 100 need to be treated with new t/t To have one less death, how many need to be treated? To have one less death, how many need to be treated? What if Risk difference is 10%; 20%; 50% What if Risk difference is 10%; 20%; 50% NNT = 100/RD (%); in decimals 1/RD NNT = 100/RD (%); in decimals 1/RD

7. A True Story Husband and pregnant wife meet their doctor Husband and pregnant wife meet their doctor Husband asks : When is the delivery expected doctor? Husband asks : When is the delivery expected doctor? Doctor : What’s the LMP Doctor : What’s the LMP Wife : 1 st March 2009 Wife : 1 st March 2009 Doctor : EDD is 8 th December 2009 Doctor : EDD is 8 th December 2009 Husband : OK, I will be here on 7 th Dec. 2009 Husband : OK, I will be here on 7 th Dec. 2009 Doctor : Oh sorry, the delivery may be earlier or later than 7 th Dec. Doctor : Oh sorry, the delivery may be earlier or later than 7 th Dec. And so on And so on

8. Degree of desired confidence level determines the width of the range (Interval) Degree of desired confidence level determines the width of the range (Interval)

9. Similarities b/w EDD range and C.I. Both need data and calculation Both need data and calculation Both capture the margin of error Both capture the margin of error Both indicate range of possibilities Both indicate range of possibilities Width of the range is directly related to the desired level of certainty Width of the range is directly related to the desired level of certainty This is only one point of true or right value in both This is only one point of true or right value in both

10. Differences b/w EDD range & C.I. EDD predicts about individual whereas C.I. about population EDD predicts about individual whereas C.I. about population No definite formula for EDD but for every C.I. there is definite formula No definite formula for EDD but for every C.I. there is definite formula Formula for C.I. differs depending on the type of data (no such for EDD) Formula for C.I. differs depending on the type of data (no such for EDD) Width of C.I. inversely related to sample size (? In EDD) Width of C.I. inversely related to sample size (? In EDD)

11. Newspaper headlines Barack Obama’s popularity rating is 55% (error +/- 10%) Barack Obama’s popularity rating is 55% (error +/- 10%) How to reduce the error? How to reduce the error? Bigger the sample size, less the error; and narrower is the width of CI Bigger the sample size, less the error; and narrower is the width of CI How much width of CI is acceptable? How much width of CI is acceptable? 45% (error +/- 10%) 45% (error +/- 10%) 65% +/- 10% 65% +/- 10%

12. Two more concepts Width of CI at a level (say 95%) depends on the sample size Width of CI at a level (say 95%) depends on the sample size Bigger the sample size, narrower the CI Bigger the sample size, narrower the CI A good width of CI is the one which yields the same answer at both its ends (limits) A good width of CI is the one which yields the same answer at both its ends (limits)

13. What is 95% confidence interval? 95% confident that the true effect is within the range 95% confident that the true effect is within the range A point in the centre is the finding from the study subjects A point in the centre is the finding from the study subjects Lines on either side denote the margin of error (range of values consistent with the data) Lines on either side denote the margin of error (range of values consistent with the data)

14. Study Q (N=400) Study D (N=4000) Risk difference 95% C.I. For two studies with small (C) and Large (D) sample size. p values approx 0.03 for both studies -50-40-30-20-1001020304050 (-1% to - 19%) (-0.1% to - 2%) Interventions Q vs Placebo Interventions P vs Placebo

15. -50-40-30-20-1001020304050 Study B (N=10,000) Study A (N=20) Risk difference 95% C.I. For two studies with small (A) and Large (B) sample size. p values 1.0 for both studies (-0.1% to + 0.1%) (-48% to + 48%) Interventions Y vs Placebo Interventions X vs Placebo

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