The Analysis of Categorical Data- Fisher's Exact Test Flashcards
1
Q
How do you calculate the P-value in a Fisher’s Exact Test compared to Chi squared test
A
Calculate the P-value DIRECTLY FROM THE DATA instead of from marginal totals equal to observed values , although not necessarily to do this when using the method
2
Q
What can you deduce from the probabilities are the same from the Fisher’s Exact Test
A
The Null hypothesis is true
3
Q
How can the P-value be found?
A
By adding up the possibilities that are LESS THAN OR EQUAL TO THE PROBABILITY OF THE OBSERVED TABLES
4
Q
What is ‘Exact’ about the the exact test?
A
- Gives a P-value that is calculated on the basis of the correct probability distribution for the different tables (no need for asymptotic approximation) assuming that the null hypothesis is true
- If performed the experiment many times with the null hypothesis true, and the MARGINS ARE FIXED AT THE OBSERVED VALUES, the P<0.05 might occur in FEWER THAN 5% OF THE EXPERIMENTS. This is NOTICED IN SMALL DATA SETS where the effects of discrepencies are inescapable. This is not a problem if the EXACT P-VALUE is quoted.
5
Q
Why Not Use the Exact Test all the time?
A
- For larger counts it would be long to process and present an algorithmic challenge
- CONFIDENCE LEVELS are WIDE and often NOT GOOD
6
Q
When should you use an exact test rather than x2 (chi squared) test?
A
- If any EXPECTED VALUE in a 2x2 table is LESS THAN 5
For larger tables
- OVER 20% of the cells have EXPECTED VALUES BELOW 5
- ANY CELLS have EXPECTED VALUE BELOW 1 then the x2 method may be unreliable
7
Q
To do a Fisher’s Test for larger values what do you need?
A
- AMALGAMATE CATEGORIES APPROPRIATELY so that the EXPECTED VALUES IN THE NEW TABLES ARE LARGER THAN THESE THRESHOLD
- STATISTICAL PACKAGES the exact test for rxc tables
(Advances have made exact tables on quite large tables entirely feasible but specialist advice is needed)
