Edward P. Sloan, MD, MPH, FACEP Basic Statistics for EM Research: Power Calculations.

Edward P. Sloan, MD, MPH, FACEP

Basic Statistics for Basic Statistics for EM ResearchEM Research::

Power Calculations Power Calculations

Edward P. Sloan, MD, MPH, FACEP

2008 UIC Research Course2008 UIC Research Course

UIC EM Residency ProgramUIC EM Residency ProgramChicago, IL Chicago, IL

September 25, 2008September 25, 2008

Edward P. Sloan, MD, MPH, FACEP

Edward P. Sloan, MD, MPH FACEPEdward P. Sloan, MD, MPH FACEP

Professor Professor

Department of Emergency MedicineDepartment of Emergency MedicineUniversity of Illinois College of MedicineUniversity of Illinois College of Medicine

Chicago, ILChicago, IL

Edward P. Sloan, MD, MPH, FACEP

Attending PhysicianAttending PhysicianEmergency MedicineEmergency Medicine

University of Illinois HospitalUniversity of Illinois Hospital

Chicago, ILChicago, IL

Edward P. Sloan, MD, MPH, FACEP

DisclosuresDisclosures

• FERNE Chairman and President

• ACEP Clinical Policy Committee

• ACEP Scientific Review Committee

• No individual financial disclosures

Power Calculations Made EasyPower Calculations Made Easy

Edward P. Sloan, MD, MPHEdward P. Sloan, MD, MPH

Associate ProfessorAssociate Professor

Department of Emergency MedicineDepartment of Emergency Medicine

University of Illinois ChicagoUniversity of Illinois Chicago

A QuestionA Question

• Current Rx: 50% admit rateCurrent Rx: 50% admit rate

• New therapy: 30% admit rate?New therapy: 30% admit rate?

• How many pts req’d to prove?How many pts req’d to prove?

• Assume: p < .05, power 80%Assume: p < .05, power 80%

A Change in OutcomeA Change in Outcome

• Current Rx: 50% admit rateCurrent Rx: 50% admit rate

• New therapy: 40% admit rate?New therapy: 40% admit rate?

• How many pts req’d to prove?How many pts req’d to prove?

• Did N change?Did N change?

A Fixed Sample SizeA Fixed Sample Size

• Current Rx: 50% admit rateCurrent Rx: 50% admit rate

• New therapy: 30% admit rate?New therapy: 30% admit rate?

• Can only include 200 patientsCan only include 200 patients

• What kind of p value to expect?What kind of p value to expect?

What is the Power?What is the Power?

• Current Rx: 50% admit rateCurrent Rx: 50% admit rate

• New therapy: 30% admit rate?New therapy: 30% admit rate?

• Can only include 200 patientsCan only include 200 patients

• What is the power?What is the power?

What Does an 83% Power Mean?What Does an 83% Power Mean?

• There is an 83% chance that a There is an 83% chance that a difference of 20% (between groups) will difference of 20% (between groups) will be detected with this sample size.be detected with this sample size.

• A difference of less than 20% could A difference of less than 20% could occur, and we won’t know it!occur, and we won’t know it!

What New Rx Proportion?What New Rx Proportion?

• Current Rx: 50% admit rateCurrent Rx: 50% admit rate

• Can only include 200 patientsCan only include 200 patients

• New therapy: what admit rateNew therapy: what admit rate?

Multiple Levels of ConfidenceMultiple Levels of Confidence

• Current Rx: 50% admit rateCurrent Rx: 50% admit rate

• New therapy: 30% admit rateNew therapy: 30% admit rate

• How much confidence that the result How much confidence that the result is “real”?is “real”?

A QuestionA Question

• URI: URI: pOpO2 2 = 94 = 94 ++ 5 5

• PE : PE : pOpO2 2 = 85 = 85 ++ 10 10

• How many pts req’d to prove?How many pts req’d to prove?

• Assume: p < .05, power 80%Assume: p < .05, power 80%

A Bigger Standard DeviationA Bigger Standard Deviation

• URI: URI: pOpO2 2 = 94 = 94 ++ 10 10

• PE : PE : pOpO2 2 = 85 = 85 ++ 10 10

• How many pts req’d to prove?How many pts req’d to prove?

• Assume: p < .05, power 80%Assume: p < .05, power 80%

Closer Mean ValuesCloser Mean Values

• URI: URI: pOpO2 2 = 94 = 94 ++ 5 5

• PE : PE : pOpO2 2 = 90 = 90 ++ 10 10

• How many pts req’d to prove?How many pts req’d to prove?

• Assume: p < .05, power 80%Assume: p < .05, power 80%

Closer Means, Bigger SDCloser Means, Bigger SD

• URI: URI: pOpO2 2 = 94 = 94 ++ 10 10

• PE : PE : pOpO2 2 = 90 = 90 ++ 10 10

• How many pts req’d to prove?How many pts req’d to prove?

• Assume: p < .05, power 80%Assume: p < .05, power 80%

ConclusionsConclusions• Know your assumptions up frontKnow your assumptions up front• Base them on the med literatureBase them on the med literature• Know your design constraintsKnow your design constraints• The power calculations are easy!The power calculations are easy!• Include the stats in the methodsInclude the stats in the methods• Discuss the clinical relevance!!!Discuss the clinical relevance!!!

Thank you.Thank you.

[email protected]@ferne.org

[email protected]@uic.edu312 413 7490312 413 7490

ferne_2008_research_lecture_sloan_stats_power_092508_final04/19/23 06:53 Edward P. Sloan, MD, MPH, FACEP