SIMPLE TWO GROUP TESTS
Prof Peter T Donnan
1. Chi-squared test (2x2)
2. t-test
3. Logrank test
CHI SQUARED TEST
Used to compare the proportions of observations in
different categories.
Example:
Achieved LDL target
Male
Gender
Female
Total
No
140
149
289
Yes
563
531
1094
Total
703
680
1383
Are gender and achieving LDL target related?
Null hypothesis: There is no association of gender
with achieving LDL target (or no difference between
men and women)
SIGNIFICANCE OF THE TEST
STATISTICS
 The value 2 = 0.834 is not significant (p = 0.361)
 The null hypothesis can not be rejected.
 Gender and achieving LDL target are not related
 % Males meeting target: 80.1%
 % Females meeting target: 78.1%
CONDITIONS
1.Data are categorical.
2.If you tried to do Crosstabs with continuous
variables you will get one column or row for
each unique value!
T-TEST – COMPARE TWO MEANS
1.Parametric test since comparing means
2.Paired samples t-test – the mean difference
between two linked groups
3.Independent samples t-test – the mean difference
between two independent groups.
PAIRED SAMPLE T-TEST
One variable measured in:
2 different groups who are matched
or same group at 2 different times (e.g before / after)
Example:
Is there a difference in LDL level before and after
treatment in the total sample?
Is this paired or unpaired?
INDEPENDENT SAMPLES T-TEST
Example: Is there a difference between the
population mean ages of the males and females?
Assumptions:
- In the population of interest the variable is
normally distributed.
- The variances of the 2 groups are the same
LOGRANK TEST
Example: Survival curves for women with glioma by diagnosis.
Bland J M , Altman D G BMJ 2004;328:1073
LOGRANK TEST
The most popular method of comparing the survival
between groups, which takes the whole follow up
period into account.
 Log rank test involves calculating Chi-squared
(2) statistic for difference in median survival
between two groups.
Example: Do colorectal cancer patients with
hypertension have worse survival than patients
without hypertension?
The Null hypothesis for the logrank test:
Hazard Rate group A = Hazard Rate for group
B
= HR = OA / EA = 1
OB / EB
THANK YOU FOR LISTENING
Explore 2-group tests further with
datasets:
LDL Data.sav
colorectal.sav