Interaction

1
Interaction

• "When the incidence rate of disease in the
  presence of two or more risk factors differs from
  the incidence rate expected to result from their
  individual effects. - MacMahon

2
Interaction

• The effect can be greater than what we would
  expect (positive interaction, synergism) or
• Less than what we would expect (negative
  interaction, antagonism)
• Should determine what we would expect to result
  from the individual effects of the exposures

3
Interaction

• Figure 15-6 An algorithm for exploring the
  possibility of interaction
• Association? Yes
• Due to confounding? No ? causal
• Equally strong in each of the strata?
• No ? Interaction present

4
Interaction

• Table 15-11 A hypothetical example
• The incidence in persons exposed
• Individual effects of each factor
• Additive model
• Attributable risks
• Table 15-12
• Table 15-13

5
Interaction

• Table 15-14 A hypothetical example
• Multiplicative model
• Relative risks
• Table 15-15
• Table 15-16

6
Interaction

• An interaction is present - when an effect is
  greater or less than what would be expected from
  the independent effects of the two separate
  factors