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  • 1. BASIC BIOSTATISTICS Diane Flynn, LTC, MC Colin Greene, LTC, MC
  • 2. ObjectivesOverview of BiostatisticalTerms and ConceptsApplication of Statistical Tests
  • 3. Why Use Statistics?Descriptive Statistics• identify patterns• leads to hypothesis generatingInferential Statistics• distinguish true differences from random variation• allows hypothesis testing
  • 4. Why Use Statistics? Cardiovascular Mortality in Males 1.2 1 0.8SMR 0.6 Bangor 0.4 Roseto 0.2 0 35- 45- 55- 65- 75- 44 54 64 74 84 AJPH 1992
  • 5. Types of DataNumerical • Continuous • DiscreteCategorical • Ordinal • Nominal
  • 6. Descriptive StatisticsIdentifies patterns in the dataIdentifies outliersGuides choice of statistical test
  • 7. Percentage of Specimens Testing Positive for RSV Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May JunSouth 2 2 5 7 20 30 15 20 15 8 4 3North- 2 3 5 3 12 28 22 28 22 20 10 9eastWest 2 2 3 3 5 8 25 27 25 22 15 12Mid- 2 2 3 2 4 12 12 12 10 19 15 8west
  • 8. Descriptive Statistics Percentage of Specimens Testing Postive for RSV 1998-99353025 South20 Northeast15 West10 Midwest 5 0 Jul Sep Nov Jan Mar May Jul
  • 9. Describing the Data with NumbersMeasures of Central Tendency • MEAN -- average • MEDIAN -- middle value • MODE -- most frequently observed value(s)
  • 10. Distribution of Course Grades 14 12 10Number of 8 Students 6 4 2 0 A A- B+ B B- C+ C C- D+ D D- F Grade
  • 11. Describing the Data with NumbersMeasures of Dispersion • RANGE • STANDARD DEVIATION • SKEWNESS
  • 12. Measures of Dispersion• RANGE • highest to lowest values• STANDARD DEVIATION • how closely do values cluster around the mean value• SKEWNESS • refers to symmetry of curve
  • 13. Measures of Dispersion• RANGE • highest to lowest values• STANDARD DEVIATION • how closely do values cluster around the mean value• SKEWNESS • refers to symmetry of curve
  • 14. Standard DeviationCurve A Curve B σB σA
  • 15. Measures of Dispersion• RANGE • highest to lowest values• STANDARD DEVIATION • how closely do values cluster around the mean value• SKEWNESS • refers to symmetry of curve
  • 16. Skewness Curve A Curve BMode Median negative skew Mean
  • 17. The Normal Distribution .Mean = median =mode Mean, Median, ModeSkew is zero68% of values fallbetween 1 SD95% of values fallbetween 2 SDs 1 2σ σ
  • 18. Inferential StatisticsUsed to determine the likelihood that aconclusion based on data from asample is true
  • 19. Termsp value: the probability that an observed difference could have occurred by chance
  • 20. Hypertension TrialDRUG Baseline mean SBP F/u mean SBP A 150 130 B 150 125
  • 21. Termsconfidence interval:The range of values we can be reasonably certain includes the true value.
  • 22. 30 Day % MortalityStudy IC STK Control p NKhaja 5.0 10.0 0.55 40Anderson 4.2 15.4 0.19 50Kennedy 3.7 11.2 0.02 250
  • 23. 95% Confidence Intervals Khaja (n=40)Anderson (n=50) Kennedy (n=250)-.40 -.35 -.30 -.25 -.20 -.15 -.10 -.05 .00 .05 .10 .15 .20
  • 24. Types of Errors Truth No Difference differenceConclusion No TYPE II difference ERROR (β ) Difference TYPE I ERROR (α) Power = 1-β
  • 25. What Test Do I Use?1. What type of data?2. How many samples?3. Are the data normally distributed?4. What is the sample size?