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1. Non-parametric tests Note: When valid use parametric
Commonly used
Wilcoxon
Chi square etc.
Performance comparable to parametric
Useful for non-normal data
If normalization not possible
Note: CI derivation-difficult/impossible
2. Wilcoxon signed rank test
To test difference between paired data
3. STEP 1 Exclude any differences which are zero
Put the rest of differences in ascending order
Ignore their signs
Assign them ranks
If any differences are equal, average their ranks
4. STEP 2
Count up the ranks of +ives as T+
Count up the ranks of –ives as T-
5. STEP 3 If there is no difference between drug (T+) and placebo (T-), then T+ & T- would be similar
If there were a difference
one sum would be much smaller and
the other much larger than expected
The smaller sum is denoted as T
T = smaller of T+ and T-
6. STEP 4 Compare the value obtained with the critical values (5%, 2% and 1% ) in table
N is the number of differences that were ranked (not the total number of differences)
So the zero differences are excluded
8. Wilcoxon rank sum test
To compare two groups
Consists of 3 basic steps
9. Non-parametric equivalent of t test
10. Step 1
Rank the data of both the groups in ascending order
If any values are equal average their ranks
11. Step 2
Add up the ranks in group with smaller sample size
If the two groups are of the same size either one may be picked
T= sum of ranks in group with smaller sample size
12. Step 3 Compare this sum with the critical ranges given in table
Look up the rows corresponding to the sample sizes of the two groups
A range will be shown for the 5% significance level