6. Contingency Analysis

Question 1

What is relative risk?

Answer 1

The probability of an outcome in the treatment group

…divided by…

the probability of
the same outcome in the control group

Question 2

What is an odds ratio?

Answer 2

An estimate of odds calculated from a random sample

P(success)/P(failure)

Question 3

how do you interpret an odds ratio

Answer 3

O > 1: success>failure

O < 1: failure > success

Question 4

how do you calculate the SE of a odds ratio?

Answer 4

SE =

Question 5

how do you calculate a confidence interval for an odds ratio

Answer 5

• use natural logs formula
• convert back using
  1/ln(x) = e^x

Question 6

how to use the chi-squared contingency test to test hypotheses

Answer 6

To calculate expected frequency under H0…
- calculate the observed probabilities for each proportion of the table

• use equation for chi-squared
• work out df by (r-1)(c-1)
• compare to chi-squared table to get p-values

Question 7

when do you use the fisher’s exact test?

Answer 7

Only use if

-2x2 table

• chi-squared cannot be used

Question 8

what does the fisher’s exact tests calculate

Answer 8

the actual p-value of a test not a test-statistic