Cohort Study

A new anti-wrinkle cream was recently developed that contains high doses of a number of chemical compounds.  As a researcher concerned with public health, you fear that some of these chemicals may be carcinogens, although the animal studies have been inconclusive.  Therefore, you decide to conduct a cohort study to determine whether the use of anti-wrinkle cream is associated with an elevated risk of skin cancer.

Question: Is anti-wrinkle cream (exposure) associated with skin cancer (outcome)?

You have an existing cohort study of individuals over the age of 60 that you have been following to look at risk factors for a number of different types of cancer including skin cancer.  Everyone was screened at the initial study visit.  All 315 individuals enrolled in your study screened negative for skin cancer at baseline (initial study visit).  Upon interviewing study participants during the baseline visit, you found that 239 of them were using anti-wrinkle cream when recruited into your study and, therefore, are classified as exposed. The remaining 76 participants (those who were not using anti-wrinkle cream at baseline) are classified as unexposed.  You follow your cohort of participants on an annual basis across 10 years.  Over the 10-year study period, 61 participants develop skin cancer and 254 participants do not.

Here is what you find in your study:

1. What is the incidence proportion (risk) of skin cancer in your study overall?
2. What is the incidence proportion (risk) of skin cancer among people who reported using anti-wrinkle cream at baseline?
3. What is the incidence proportion (risk) of skin cancer among people who reported not using anti-wrinkle cream at baseline?
4. What is the risk ratio of skin cancer for people using anti-wrinkle cream compared to people not using the cream at baseline? Interpret this number in words.
5. Since you followed the individuals for 10 years, people who got skin cancer got it at different points in time. You therefore decide it would be better to calculate incident rates instead of incident proportions.  You calculate person time at risk for each of your groups:

• The individuals who reported using anti-wrinkle cream at baseline contributed 2227 person years of observation at risk.
• The individuals who reported not using anti-wrinkle cream at baseline contributed 680 person years of observation at risk.
  1. What is the incidence rate of skin cancer in your study overall?

1. What is the incidence rate among people who reported using anti-wrinkle cream at baseline?
2. What is the incidence rate among people who reported not using anti-wrinkle cream at baseline?
3. What is the rate ratio of skin cancer comparing people using anti-wrinkle cream at baseline to people not using the cream? Interpret this number in words.

6. Based on the results of this cohort study, what do you conclude about the association between anti-wrinkle cream (exposure) and skin cancer (disease)? Can you say definitively that the anti-wrinkle cream is associated (either protective or harmful) with skin cancer? Why or why not?