Researchers enrolled 5000 head injury-free cyclists into a prospective cohort st

Question

Researchers enrolled 5000 head injury-free cyclists into a prospective cohort study. “Helmet use” was defined as self-reported helmet use during at least 90% of their time cycling. Once a year, participants were contacted and asked if they had a head injury due to a bike crash in the past year. All reported head injuries were confirmed with medical records, and only those that were confirmed were included in analysis. The participants were followed for 4 years.

1. Calculate the cumulative incidence of head injury during the follow-up period for helmet users and non-users (separately for each group).

2. Calculate the relative risk for head injury associated with helmet use (helmet use should be defined as the “exposed” group).

HEAD INJURY INCIDENCE AMONG HELMET USERS AND NON-USERS (N 5,000) Total #with head injury during follow-up 152 163 # without head injury during follow- up 3,598 1,087 # person-years observed during follow- up 13,688 3,375 Group Helmet Users Helmet Non-Users 3750 1250

Explanation / Answer

Question 1

Cumulative incidence of head injury during follow up period for helmet users = 152/3750 = 0.0405

Cumulative incidence of head injury during follow up period for no- helmet users = 163/1250 = 0.1304

(2)

Here Relative Risk for head injury associated with helmet use (Here helmet use is exposed group) = Iexposed /Inexpod = 0.0405/0.1304 = 0..31

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