Bite Size Ebm Kameshwar Prasad Sept 2009

8/8/2019 Bite Size Ebm Kameshwar Prasad Sept 2009

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Which Treatment ?Which Treatment ?

For a patient, 64For a patient, 64--yearyear--old maleold male

No HT, DM, Heart failure, LV dysfunctionNo HT, DM, Heart failure, LV dysfunction

Having nonHaving non--rheumaticrheumatic atrialatrial fibrillationfibrillation Treatment A reduces annual stroke risk by 40%Treatment A reduces annual stroke risk by 40%

Treatment B requires 250 patients to be treatedTreatment B requires 250 patients to be treated

for one year to prevent one strokefor one year to prevent one stroke

Treatment C reduces annual stroke risk fromTreatment C reduces annual stroke risk from

1.0% to 0.6%1.0% to 0.6%


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Newspaper headlines: EconomicNewspaper headlines: Economic

reforms succeedreforms succeed Economic reforms led to decline inEconomic reforms led to decline in

unemployment from 20% to 15%, thusunemployment from 20% to 15%, thus

reducing it by 25%reducing it by 25%


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Newspaper headlines: EconomicNewspaper headlines: Economic

reforms succeedreforms succeed Economic reforms led to decline inEconomic reforms led to decline in

unemployment from 20% to 15%, thusunemployment from 20% to 15%, thus

reducing it by 25%.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%


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Different ways of expressing effectsDifferent ways of expressing effects

Economic reforms led to decline inEconomic reforms led to decline inunemployment rate from 20% to 15%,unemployment rate from 20% to 15%,

thus reducing it by 25%thus reducing it by 25% 20%20% -- 15% = 5%15% = 5%

If 20% = 100%If 20% = 100%

Then 15% = 75%Then 15% = 75%And 5% = 25%And 5% = 25%

Baseline risk

Relative riskRelative risk reduction

Absolute risk reduction


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In an Intervention trialIn an Intervention trial

Intervention led to decline in riskIntervention led to decline in risk(incidence) of mortality from 20% (in(incidence) of mortality from 20% (incontrol group) to 15% (in interventioncontrol group) to 15% (in intervention

group), thus reducing it by 25%.group), thus reducing it by 25%. Risk Difference is the simple differenceRisk Difference is the simple difference

between the two risks = 5%.between the two risks = 5%.

If 20% is taken as 100%, thenIf 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 (100And 5% = 25% i.e. RRR (100--75 i.e. 10075 i.e. 100--

RR (%) or in decimals, 1RR (%) or in decimals, 1 -- RRRR


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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 treated5 less death per 100 need to be treatedwith new t/twith new t/t

To have one less death, how many needTo have one less death, how many need

to be treated?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/RDNNT = 100/RD (%); in decimals 1/RD


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A True StoryA True Story

Husband and pregnant wife meet their doctorHusband and pregnant wife meet their doctor

Husband asks : When is the delivery expectedHusband asks : When is the delivery expected

doctor?doctor?

Doctor : Whats the LMPDoctor : Whats the LMP Wife : 1Wife : 1ststMarch 2009March 2009

Doctor : EDD is 8Doctor : EDD is 8thth December 2009December 2009

Husband : OK, I will be here on 7Husband : OK, I will be here on 7thth Dec. 2009Dec. 2009 Doctor : Oh sorry, the delivery may be earlier orDoctor : Oh sorry, the delivery may be earlier or

later than 7later than 7thth Dec.Dec.

And so onAnd so on


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Degree of desired confidence levelDegree of desired confidence level

determines the width of the rangedetermines the width of the range(Interval)(Interval)


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Similarities b/w EDD range and C.I.Similarities b/w EDD range and C.I.

Both need data and calculationBoth need data and calculation

Both capture the margin of errorBoth capture the margin of error

Both indicate range of possibilitiesBoth indicate range of possibilities

Width of the range is directly related toWidth of the range is directly related to

the desired level of certaintythe desired level of certainty This is only one point of true or right valueThis is only one point of true or right value

in bothin both


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Differences b/w EDD range & C.I.Differences b/w EDD range & C.I.

EDD predicts about individual whereas C.I.EDD predicts about individual whereas C.I.about populationabout population

No definite formula for EDD but for everyNo definite formula for EDD but for everyC.I. there is definite formulaC.I. there is definite formula

Formula for C.I. differs depending on theFormula for C.I. differs depending on thetype of data (no such for EDD)type of data (no such for EDD)

Width of C.I. inversely related to sampleWidth of C.I. inversely related to samplesize (? In EDD)size (? In EDD)


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Newspaper headlinesNewspaper headlines

Barack Obamas popularity rating is 55%Barack Obamas popularity rating is 55%(error +/(error +/-- 10%)10%)

How to reduce the error?How to reduce the error? Bigger the sample size, less the error; andBigger the sample size, less the error; and

narrower is the width of CInarrower is the width of CI

How much width of CI is acceptable?How much width of CI is acceptable? 45% (error +/45% (error +/-- 10%)10%)

65% +/65% +/-- 10%10%


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Two more conceptsTwo more concepts

Width of CI at a level (say 95%) dependsWidth of CI at a level (say 95%) dependson the sample sizeon the sample size

Bigger the sample size, narrower the CIBigger the sample size, narrower the CI

A good width of CI is the one which yieldsA good width of CI is the one which yieldsthe same answer at both its ends (limits)the same answer at both its ends (limits)


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What is 95% confidenceWhat is 95% confidence

interval?interval? 95% confident that the true effect is95% confident that the true effect is

within the rangewithin the range

A point in the centre is the finding fromA point in the centre is the finding fromthe study subjectsthe study subjects

Lines on either side denote the margin ofLines on either side denote the margin oferror (range of values consistent with theerror (range of values consistent with thedata)data)


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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 -10 0 10 20 30 40 50

(-1% to - 19%)

(-0.1% to - 2%)Interventions Q vs Placebo

Interventions P vs Placebo


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-50 -40 -30 -20 -10 0 10 20 30 40 50

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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THANK YOUTHANK YOU

Bite Size Ebm Kameshwar Prasad Sept 2009

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