Question

	Helmet worn	No Helmet
Facial injuries received	30	182
All injuries nonfacial	83	236

For the group that did not wear helmets, find the probabilty of randomyl selecting a subject from this study and getting someone who had facial injuries. Can this probability be used to estimate the incidence of facial injuries among those who do not wear helmets?

Answer 1

	Helmet worn	No Helmet	Total
Facial injuries received	30	182	212
All injuries non-facial	83	236	319
Total	113	418	531

P(Facial injuries received | No Helmet) = P(Facial injuries received and No Helmet) / P(No Helmet)

= (182/531) / (418/531) = 0.4354

The design is that ot retrospective study selected by the researcher, it is not representative ot the general. So the results obtain trom this sample cannot be generalized, so the probability value is not used an estimate ot the actual incidence rate.