7. THE ODDS RATIO

Question 1

1. Fill in the labels for the spaces numbered 1 and 2.

Answer 1

1. Measures of Association
2. Measures of Frequency

Question 2

2. Fill in the labels for the spaces numbered 1-4.

Answer 2

1. Prevalence
2. Point
3. Period
4. Odds

Question 3

3. Fill in the labels for the spaces numbered 5-7.

Answer 3

5. Incidence
6. Cumulative
7. Rate

Question 4

4. What is the Odds?

Answer 4

• this is a ratio of 2 odds
• it is an alternative way of measuring frequency

Question 5

5. How do we calculate the Odds?

Answer 5

• the numerator is the people with the existing disease
• the denominator is the people without the disease

Question 6

6. What does Odds compare?

Answer 6

• it is used for assessing the likelihood of having an
  outcome on interest (such as a disease)
• the denominator is the number of non-cases
  (non diseased)
• instead of having the whole population as the
  denominator

Question 7

7. What does Odds measure?

Answer 7

• it measures the likelihood of having the outcome
  IN RELATION to the probability of not having the
  outcome

EXAMPLE:
- the odds of being affected by the disease VERSUS the
odds of not being affected by the disease

Question 8

8. Is the Odds commonly used?

Answer 8

• no
• it is not often used on its own as a measure of disease
  frequency
• it is very commonly used for calculating the Odds Ratio

Question 9

9. Fill in the labels for the spaces labelled 1-3.

Answer 9

1. Odds Ratio
2. Risk Ratio
3. Rate Ratio

NB:
- we use the Odds Ratio when we cannot ascertain new
cases

Question 10

10. When is the Odds Ratio used instead of Relative Risk?

Answer 10

• sometimes it is impossible to investigate an association
  prospectively
• this is when we assess the exposure at baseline
• we then have follow up participants that ascertain
  new disease cases

Question 11

11. When is it impossible to investigate an association prospectively?

Answer 11

• when there is a lack of financial resources
• when there is a lack of other resources
• when there is limited time availability
• when the outcome condition of the interest is very rare
  in the population

Question 12

12. What do we do when the Prospective Investigation is not possible?

Answer 12

THE ASSOCIATION IS INVESTIGATED:
1. Cross-Sectionally
2. Retrospectively

Question 13

13. What is a Cross Sectional Investigation?

Answer 13

• the exposure and the outcome are assessed at the
  same point in time

Question 14

14. What is a Retrospective Investigation?

Answer 14

• this is when we make use of existing outcome cases
  and go back in time to assess the exposure

Question 15

15. In which two cases is the Odds Ratio used?

Answer 15

• it is used when we analyse an association Cross-
  Sectionally or Retrospectively
• this is because we cannot calculate the incidence (risk)
• this means that we cannot calculate the Relative Risk

NB:
- the Odds Ratio is an alternative measure of association
- it is used in Retrospective studies

Question 16

16. What are the 3 ways of Interpreting the Odds Ratio value?

Answer 16

1. The value is greater than 1
2. The value is less than 1
3. The value is equal to 1

NB:
- the Odds Ratio is very similarly interpreted to the Risk
Ratio

Question 17

17. What does it mean when the Odds Ratio Value is greater than 1?

Answer 17

• the exposed group have higher odds (likelihood) of the
  outcome (disease) than the unexposed group
• the exposure is harmful for the outcome

Question 18

18. What does it mean when the Odds Ratio Value is less than 1?

Answer 18

• the exposed group have lower odds (likelihood) of the
  outcome (disease) than the unexposed group
• the exposure is protective for the outcome

Question 19

19. What does it mean when the Odds Ratio Value is equal to 1?

Answer 19

• the exposed group have exactly the same
  odds (likelihood) of the outcome (disease) as the
  unexposed group
• the exposure has no association with the outcome