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Basic Statistics: The Three Things You Need to Know In Order to Complete Your Project

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Edward P. Sloan, MD, MPH Associate Professor Dept of Emergency Medicine University of Illinois College of Medicine Chicago, IL

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Attending Physician Emergency Medicine University of Illinois Hospital Our Lady of the Resurrection Medical Center

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Overview Global Objectives Answer clinical questions Learn minimal statistics Get it done

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Overview Session Objectives Know what question to ask Review stats needed to answer question Collect data for stat analysis

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Three Things to Know What measurement? What question? How to report your results

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Other Things to Know What data is needed? Basic statistical tests Nothing else

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Two Measurements Proportion Mean

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Two Questions What is the number? Do two groups differ?

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Proportion Measurement What % of patients had some outcome? Divides a group into subgroups

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Mean Measurement Describes a characteristic of a patient group Provides an overview

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What is the Number? Describes one group Tells a story No comparisons

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What #? Confidence Interval Can we believe the results? Was the process acceptable? Are the results reproducible?

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Confidence Interval 95% CI Provides a range The “real” number is in this range

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Do Two Groups Differ? Compares two groups Attempts to detect differences Relates outcomes to different interventions

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Groups Differ? Hypothesis Test Null hypothesis, H o Alternate hypothesis, H A Are the groups the same (H o ) or different (H A )?

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Hypothesis Test We reject H o The groups aren’t the same We then accept that they differ

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Using Statistics 1)Pick your number type (Proportion or mean) 2)Decide what you will do (Describe or compare) (CI or hypothesis test) 3)Do the stat analysis 4)Report the results

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Reporting Proportions 95% Confidence Interval In patients with T-wave inversion on their ED EKG, 29 (+ 3)% will develop an AMI during hospitalization (95% CI 26-32%).

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Reporting Proportions: Hypothesis Test There was a two-fold greater 5 year survival rate in drug A patients vs. drug B patients (30% vs. 15%, p<.001)

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Reporting Means: 95% Confidence Interval The mean CPK-MB in ED T-wave inversion AMI patients was (95% CI=9-15).

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Reporting Means: Hypothesis Test Drug A patients had a 50% greater survival time than that of drug B patients ( years vs years, p<.001).

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Statistics: Proportions Chi-square Fisher’s Exact test

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Statistics: Means Students’s t-test ANOVA

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Key Concepts: Design Know your number Know what question you’ll do Know how you will report results

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Key Concepts: Data Collection Data to describe groups Data to compare groups Data to provide results Data to study confounding

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Key Concepts: Data Analysis Simple data elements Ability to do 2x2 analysis Ability to do simple regression

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Key Concepts: Reporting Consistent statements Straightforward p-value is less important

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Statistics: Reporting Results Differences in outcome Absolute vs. Relative Mortality 25% to 20% –5% absolute decrease –20% relative decrease

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Statistics: Reporting Results Odds ratios Relative risk Regression analysis What variables predict outcome?

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Example: 2x2 Analysis Risk variable Outcome measure OR, 95% CI p-value Conclusion

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Conclusions Know baseline proportions/means Project your results Know what data is needed Collect the data Analyze via 2x2 tables

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Recommendations Keep it simple Follow our directions completely

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