3. RISK/RATE RATIO - Measures Of Association

Question 1

What is exposure?

Answer 1

• An event/state/behaviour (i.e. any variable) to which a group of people is
  ‘exposed’

Question 2

What is outcome?

Answer 2

• An event/state/behaviour (i.e. any variable), which we hypothesize has resulted from exposure to some other event/state/behaviour (i.e. anexposure)

Question 3

What does outcome measure?

Answer 3

Frequency

Question 4

Measure of association breaks down into

Answer 4

Binary and numeric is outcomes

Binary outcomes consist of 3 ratios:
Odds ratio - existing cases
Risk ratio - new cases
Rate ratio - new cases

Numeric outcomes consistent of 3 things:
Mean difference - categorical exposure
Correlation coefficient - numeric exposure
Regression coefficient - number is exposure

Question 5

What does “Measures of Association” mean?

Answer 5

Assess the presence and the magnitude of an association between an exposure (IV) and an outcome (DV)

• Asses the change in outcome for a given change in exposure
• Apply to both numeric and categorical exposures
  and outcomes

Question 6

What is relative risk?

Answer 6

It’s the comparison between two risks (incidences); or better refers to a risk in relation to another
(reference) risk

• Relative Risk is a measure of association and as such it
  assesses the probability of an event occurring (i.e. the risk
  of outcome) in an exposed group of people relative/ compared to the probability of the same event occurring in an unexposed group of people (reference)
• Thus both exposure and outcome variables are categorical

Example of an exposure and its outcome: smoking = lung cancer.

Question 7

What two measures of association is used for relative risk?

Answer 7

Risk Ratio: is a ratio of two risks (cumulative incidences)
Rate Ratio or Hazard Ratio is a ratio of two rates (incidence rates)

Question 8

What are the exposure categories in relative risk?

Answer 8

• exposed category
• Unexposed or reference category

Two groups are compared where one of the 2 are either of these.

For example:

Exposure: smoking status
Exposure categories: smokers (exposed); non-smokers (unexposed)

Exposure: medication type
Exposure categories: active medication (exposed group); placebo (unexposed group)

Question 9

What is important to remember when calculating relative risk?

Answer 9

• When calculating a Relative Risk, the exposed
  category is always compared to the unexposed
  (reference) category, not vice versa!
• When there are more than 2 groups one group must be the unexposed category while the others are exposed.
  Exposure: alcohol consumption
  Exposure categories: heavy (exposed), moderate (exposed), light (unexposed)
• Both the Risk Ratio and Rate Ratio are applicable in
  situations where there is follow-up (i.e.
  prospective studies) and ascertainment of new
  (rather than existing) outcome cases

Question 10

RISK Ratio formula

Answer 10

Numerator (exposed): NEW CASES over specific period/total population.
Denominator (unexposed): NEW CASES over a specific period/total population.

Question 11

RATE Ratio formula

Answer 11

Numerator (exposed): NEW CASES over specific period/total person-time of follow up
Denominator (unexposed): NEW CASES over a specific period/ total person time of follow-up.

Question 12

Relative Risk Example: A prospective study investigated the associated between smoking (exposure) and lung cancer (outcome)

Answer 12

3.85, risk ration is usually presented in 2 decimal places.

Question 13

Important to remember!

Answer 13

1. Prevalence = percentage
2. Incidence = person-years
3. Risk Ratio = -1 *100 (percentage)
4. Rate Ratio = 2 DP (-fold)
5. Odds = divided by NON-CASES

Question 14

What are the three ways you can interpret relative risk?

Answer 14

> 1: the exposed have higher risk (or probability) of the
outcome (disease) than the unexposed
(i.e. the exposure is harmful for the outcome!)

<1: the exposed have lower risk (or probability) of the
outcome (disease) than the unexposed
(i.e. the exposure is protective for the outcome!)

=1: the exposed have exactly the same risk (or probability) of the outcome (disease) with the unexposed
(i.e. the exposure has no association with the outcome!)

Question 15

Why is rate ratio more accurate than risk ratio?

Answer 15

Rate Ratios are considered more accurate, since they take into account the real follow-up time.

Question 16

Odds formula

Answer 16

Question 17

What is odds?

Answer 17

• An alternative measure for the likelihood of having
  an outcome of interest (e.g. disease) which instead of
  using the whole population as the denominator,
  uses the non-cases (i.e. non-diseased)
• Odds measures the likelihood of having the outcome
  in relation to the probability of not having the outcome,
  i.e. odds of being affected (by disease) versus odds
  of not being affected.
• Odds is not commonly used on its own as a measure
  of disease frequency, but it is very commonly used for
  calculating the Odds Ratio (see next slides)

Question 18

When is Odds ratio used instead of Relative risk?

Answer 18

When it investigates association prospectively due to:
– lack of financial and other resources
– limited time availability
– outcome condition of interest is very rare in the
population

Question 19

How do we interpret the relative risk of 3.85?

Answer 19

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