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Incidence and incidence ratios of favism (crude)
GPDH deficiency
Present 2.03%
(2,030 cases / 100,000 people)
Absent 0.03%
(2,970 / 9,900,000 people)
Incidence ratio 67.67
Eat fava beans
Yes 0.13%
(2,600 cases / 2,000,000)
No 0.03%
(2,400 / 8,000,000)
Incidence ratio 4.33
So indeed, the scarcer factor (GPDH deficiency) has the greater incidence ratio. If we increase the
prevalence of GPDH deficiency without changing other parameters, the incidence ratio for fava
bean consumption will rise. A spreadsheet is a convenient way to see the effect on incidence ratios
from varying the prevalences (check the web page for a downloadable Excel spreadsheet).
Bottom line – what we observe as strength of association is greatly dependent upon prevalence of
other component causes.
The above example also illustrates the non-additivity of the attributable risk proportion
[ARP=(RR-1)/RR]):
67.67 – 1
ARP for GPDH deficiency –––––––––– = 98.5 %
67.67
4.33 – 1
ARP for Fava bean consumption –––––––––– = 76.9 %
4.33
Clearly, these ARP's do not sum to 100%, nor, when we think about it, should they.
Before continuing with Rothman's diagrams, we need to revisit an old friend, weighted averages.
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www.epidemiolog.net, © Victor J. Schoenbach 12. Multicausality: Effect modification - 393
rev. 11/5/2000, 11/9/2000, 5/11/2001