Critiquing for Evidence-based Practice: Therapy or Prevention

Critiquing for Evidence-based Practice: Therapy or Prevention

M8120Columbia University

Suzanne Bakken, RN, DNSc

Critical Analysis of Therapy or Prevention Studies

Are the results of the study valid?

What are the results?Will the results help me in caring

for my patients?


Are the results of the study valid?– Was the assignment of patients to

treatment randomized?


Are the results of the study valid?– Were all patients who entered the

trial properly accounted for and attributed at its conclusion?–Follow-up complete– Intent to treat analysis


Are the results of the study valid?– Were patients, their clinicians,

study personnel “blind” to treatment?


Are the results of the study valid?– Were the groups similar at the

start of the trial?


Are the results of the study valid?– Aside from the experimental

intervention, were the groups treated equally?




for my patients?


What are the results?Focus on clinical rather than

statistical significance?– How large was the treatment effect?– How precise was the estimate of the

treatment effect?– RR, RRR, CI, etc.


Risk without therapy (Baseline)= X Risk with therapy = Y Absolute Risk Reduction(ARR) (risk difference) = X - Y Relative risk (RR) = Y/X Relative risk reduction (RRR)=X-Y (ARR)/X (baseline) *

100 Confidence interval (CI) = range that includes the true

RRR a set percentage of time


Death = 20% in control group and 15% in experimental

How large was the treatment effect?– Risk without therapy (Baseline)= X– Risk with therapy = Y– Absolute Risk Reduction(ARR) (risk difference) = X -

Y– Relative risk (RR) = Y/X– Relative risk reduction (RRR)=X-Y (ARR)/X

(baseline) * 100


Target disorder – Type I diabetes Rx – intensive insulin treatment Event prevented (Diabetic retinopathy) = 9.6% in

control group and 2.8% in experimental How large was the treatment effect?

– Risk without therapy (Baseline)= X– Risk with therapy = Y– Absolute Risk Reduction(ARR) (risk difference) = X -

Y– Relative risk reduction (RRR)=X-Y (ARR)/X


Target disorder – Type I diabetes Rx – intensive insulin treatment Event prevented (Diabetic retinopathy) = 9.6% in

control group and 2.8% in experimental How large was the treatment effect?

– Risk without therapy (Baseline)= X=.096– Risk with therapy = Y=. 028– Absolute Risk Reduction(ARR) (risk difference) = X

– Y = .068– Relative risk reduction (RRR)=X-Y (ARR)/X = .71


Target disorder – independent elderly Rx – Comprehensive geriatric assessment Event prevented (long term nursing home admission)

= 10% in control group and 4% in experimental How large was the treatment effect?

– Risk without therapy (Baseline)= X– Risk with therapy = Y– Absolute Risk Reduction(ARR) (risk difference) = X -

Y– Relative risk reduction (RRR)=X-Y (ARR)/X




for my patients?


Will the results help me in caring for my patients?– Are the results applicable to my

patient?– Were all clinically important

outcomes considered?– Are the likely treatment benefits

worth the potential harm and costs?


Will the results help me in caring for my patients?– Are the results applicable to my

patient?


Will the results help me in caring for my patients?– Were all clinically important

outcomes considered?


Will the results help me in caring for my patients?– Are the likely treatment benefits

worth the potential harm and costs (number needed to treat)?


Number needed to treat– 1/ARR– Benefits AND risks (including costs)

of treatment depend on:–RRR–Risk of adverse outcome it is

designed to prevent (i.e., prior probability)


Ezekowitz et al, 1992 - Warfarin Rx with nonvalvular atrial fibrillation– Baseline risk of stroke = 0.043– Risk of stroke with Rx = 0.009– ARR = 0.043-0.009 = 0.034– NNT to prevent 1 stroke = 1/0.034= 30


Importance of baseline risk (prior probability) - Example: beta blocker after MI– Baseline risk = 0.01 vs. 0.10– RR with therapy = 0.75– ARR = ? And ?– NNT = ? And ?


Diastolic BP 115-129– Baseline risk of death, stroke or MI =

13%– Risk of death, stroke or MI with Rx =

1.4%– NNT=


Diastolic BP 90-109– Baseline risk of death, stroke or MI =

5.5%– Risk of death, stroke or MI with Rx =

4.7%– NNT=

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Critiquing for Evidence-based Practice: Therapy or Prevention