Question

Text: Scenario #1 - Prospective Cohort Study Studies published in the scientific literature have consistently demonstrated that wearing a helmet while bicycling is associated with lower odds of head injury in a crash. However, there has been criticism of these studies in the media, including the assertion that these studies have overstated the protective effect of helmets. One of the most common critiques includes the frequent use of the case-control study design and limitations associated with this design. One of the primary reasons that case-control studies have been used is because head injuries attributable to bike crashes are a relatively rare event. Cohort studies, on the other hand, can provide stronger evidence due to their prospective nature, but they require far more time and resources than a case-control study, especially for rare outcomes. 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 incident head injuries were confirmed with medical records, and only those that were confirmed were included in the analysis. The participants were followed for 4 years. HEAD INJURY INCIDENCE AMONG HELMET USERS AND NON-USERS (N=5,000) Group # with head injury during follow-up # without head injury during follow-up Total # person-years observed during follow-up Helmet Users 152 3,598 3750 13,688 Helmet Non-Users 163 1,087 1250 3,375 Calculate the relative risk for head injury associated with helmet use (helmet use should be defined as the "exposed" group) based on cumulative incidence (not using person years). Enter as '#.##'. (1 point) Calculate the relative risk of head injury associated with helmet use (helmet use should be defined as the "exposed" group) using incidence density (using person years). Enter as '#.##' (1 point). Which of these incidence measures (cumulative incidence or incidence density) provides the most accurate estimate of risk? (1 point) Cumulative Incidence Incidence Density Hypothetically suppose that the relative risk for head injury is 0.55 comparing helmet users to non-users (this is not the relative risk that you calculated above), interpret the relative risk estimate. (1 point) Helmet users have 0.55 times the risk of head injury as compared to non-helmet users. Helmet users are 0.55 times as likely to have a head injury as compared to non-helmet users. Helmet users are 45% less likely (have 45% lower risk) of head injury compared to non-helmet users. All of the above.

Text: Scenario #1 - Prospective Cohort Study

Studies published in the scientific literature have consistently demonstrated that wearing a helmet while bicycling is associated with lower odds of head injury in a crash. However, there has been criticism of these studies in the media, including the assertion that these studies have overstated the protective effect of helmets. One of the most common critiques includes the frequent use of the case-control study design and limitations associated with this design. One of the primary reasons that case-control studies have been used is because head injuries attributable to bike crashes are a relatively rare event. Cohort studies, on the other hand, can provide stronger evidence due to their prospective nature, but they require far more time and resources than a case-control study, especially for rare outcomes.

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 incident head injuries were confirmed with medical records, and only those that were confirmed were included in the analysis. The participants were followed for 4 years.

HEAD INJURY INCIDENCE AMONG HELMET USERS AND NON-USERS (N=5,000)

Group
# with head injury during follow-up
# without head injury during follow-up
Total
# person-years observed during follow-up
Helmet Users
152
3,598
3750
13,688
Helmet Non-Users
163
1,087
1250
3,375

Calculate the relative risk for head injury associated with helmet use (helmet use should be defined as the "exposed" group) based on cumulative incidence (not using person years). Enter as '#.##'. (1 point)

Calculate the relative risk of head injury associated with helmet use (helmet use should be defined as the "exposed" group) using incidence density (using person years). Enter as '#.##' (1 point).

Which of these incidence measures (cumulative incidence or incidence density) provides the most accurate estimate of risk? (1 point)

Cumulative Incidence
Incidence Density

Hypothetically suppose that the relative risk for head injury is 0.55 comparing helmet users to non-users (this is not the relative risk that you calculated above), interpret the relative risk estimate. (1 point)

Helmet users have 0.55 times the risk of head injury as compared to non-helmet users.
Helmet users are 0.55 times as likely to have a head injury as compared to non-helmet users.
Helmet users are 45% less likely (have 45% lower risk) of head injury compared to non-helmet users.
All of the above.

Added by M-Nica M.

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