1. BASIC BIOSTATISTICS
Diane Flynn, LTC, MC
Colin Greene, LTC, MC

2. Objectives
Overview of Biostatistical
Terms and Concepts
Application of Statistical Tests

3. Why Use Statistics?
Descriptive Statistics
• identify patterns
• leads to hypothesis generating
Inferential Statistics
• distinguish true differences from
random variation
• allows hypothesis testing

4. Why Use Statistics?
Cardiovascular Mortality in Males
1.2
1
0.8
SMR 0.6 Bangor
0.4 Roseto
0.2
0
'35- '45- '55- '65- '75-
'44 '54 '64 '74 '84
AJPH 1992

5. Types of Data
Numerical
• Continuous
• Discrete
Categorical
• Ordinal
• Nominal

6. Descriptive Statistics
Identifies patterns in the data
Identifies outliers
Guides choice of statistical test

7. Percentage of Specimens Testing
Positive for RSV
Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun
South 2 2 5 7 20 30 15 20 15 8 4 3
North- 2 3 5 3 12 28 22 28 22 20 10 9
east
West 2 2 3 3 5 8 25 27 25 22 15 12
Mid- 2 2 3 2 4 12 12 12 10 19 15 8
west

8. Descriptive Statistics
Percentage of Specimens Testing Postive for
RSV 1998-99
35
30
25 South
20 Northeast
15 West
10 Midwest
5
0
Jul Sep Nov Jan Mar May Jul

9. Describing the Data
with Numbers
Measures of Central Tendency
• MEAN -- average
• MEDIAN -- middle value
• MODE -- most frequently observed
value(s)

10. Distribution of Course Grades
14
12
10
Number 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 Numbers
Measures 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 Deviation
Curve 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 B
Mode
Median
negative
skew
Mean

17. The Normal Distribution
.
Mean = median =
mode
Mean, Median, Mode
Skew is zero
68% of values fall
between 1 SD
95% of values fall
between 2 SDs
1 2σ
σ

18. Inferential Statistics
Used to determine the likelihood that a
conclusion based on data from a
sample is true

19. Terms
p value: the probability that an observed
difference could have occurred by
chance

20. Hypertension Trial
DRUG Baseline mean SBP F/u mean SBP
A 150 130
B 150 125

21. Terms
confidence interval:
The range of values we can be
reasonably certain includes the true
value.

22. 30 Day % Mortality
Study IC STK Control p N
Khaja 5.0 10.0 0.55 40
Anderson 4.2 15.4 0.19 50
Kennedy 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
difference
Conclusion 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?