1. David Pieper, Ph.D. dpieper@med.wayne.edu
STATISTICS
David Pieper, Ph.D.

2. Types of Variables Categorical Variables
Organized into category
No necessary order
No quantitative measure
Examples
male, female
race
marital status
treatment A and treatment B

3. Types of Variables Continuous Variables
Have specific order
Examples:
weight
temperature
blood pressure
Age
Test score
May be converted to categorical

4. Descriptive Statistics
Measures of central tendency
mean (average)
Measures of variability
range
standard deviation

5. Results of Memory Test Age Gender Age Group Student or Parent
Total Score 17 M HS S 52 16 49 30 F
Adult P 50 47 43 41 36 51 33

6. Descriptive Statistics for Memory Test
Age
Total Score
Number of Cases
309
Minimum 7 12
Maximum 72 54
Mean
24.9
38.0 SD 16 8

7. Research Hypothesis
Null hypothesis: relationship among phenomena does not exist
Example: Age does not have an influence on memory

8. Probability and p Values
1 in 20 or 5% chance groups are not different when we say groups are significantly different
p < 0.01
1 in 100 or 1% chance of error
p < 0.001
1 in 1000 or .1% chance of error

9. Type of Statistical Test to Use
Continuous variable as end point
2 groups: t-test
3 or more groups: ANOVA
Relation between 2 categorical variables:
Chi-square test
Relation between 2 continuous variables:
Regression analysis or correlation

10. T-test When comparing 2 groups and end-point variable is continuous
Purpose is to determine if the difference between the 2 groups is unlikely due to chance

11. T-test Examples: Blood pressure before and after exercise program
Would parents do better on a memory test than students

12. T-test results comparing Parents and Students Total Score
Number
Mean SD
Students
180
37.0
8.0
Parents
129
39.5
7.9
p < 0.05
Parents had higher scores than students

13. Analysis of Variance (ANOVA)
When comparing 3 or more groups and end-point is continuous
Example: Compare score on memory test among:
Grade school students
Middle school students
High school students
Parents

14. Total Score Analysis of Variance p < 0.001
High School Students and Adults scored better than Grade School or Middle School Students
Middle School Students scored better than Grade School Students

15. Chi-square Test
When comparing 2 or more groups and the end point is categorical

16. Chi-square Gender and Parent vs Student
Total
Female
101
(56%) 77
(60%)
142
Male
79 (44%) 52
(40%)
103
140
105
245
Percent
P = 0.6
There was no significant gender difference between students and parents

17. Correlation or Regression
When determining if there is a linear relationship between 2 continuous variables
Ranges from -1 to 1

18. Pearson’s Correlation Coefficient
Is Diastolic BP related to Weight?
r = p < 0.01

19. (mm Hg)
(kg) (

20. Correlation of Age and Score on Memory Test
No correlation of age and score on memory test

21. Illustrations: Use Graphs
Label axes
Include brief
description
p < 0.01
Patients that failed the exercise test had a higher mortality than patients that passed

22. Free Statistics Software
Systat:
List of Free Statistics Software: