Essentials of Epidemiology in Public Health, Fourth Edition Ann Aschengrau and George Seage In-Class Exercise​​ ​​ In-Cl

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Essentials of Epidemiology in Public Health, Fourth
Edition
Ann Aschengrau and George Seage
In-Class Exercise​​
​​
In-Class Exercise for Small Groups
Confounding and Random Error
​
Physical activity is the centerpiece of many strategies to prevent
and reduce chronic disease and improve health. The US Department of
Health and Human Services (HHS) sets Physical Activity Guidelines
for Americans based on the current body of evidence. Adults are
encouraged to engage in 2 hours and 30 minutes of moderate
intensity physical activity per week; children and adolescents are
encouraged to participate in 60 minutes of physical activity
daily.
Suppose that we work for HHS, which is interested in understanding
how body size and physical activity during different periods in
adolescence may impact health. We are tasked with critically
evaluating a recent study that examined the association between
adolescent body size, physical activity, and risk of breast cancer
(Oh et al. Interaction between early life body size and physical
activity on risk of breast cancer. Int J Cancer 2015; 137:571-581).

This investigation was a prospective cohort study of female nurses
aged 25-42 who were free of breast cancer at enrollment in 1989. In
1997, participants were asked to report their childhood and
adolescent body size (at ages 5, 10, and 20) using a 9-level figure
drawing (1 being very thin and 9 being overweight). Participants
have been followed-up since 1989 for incidence of breast
cancer.

1. Based on the description in the introductory paragraph, which
potential confounding variables were controlled through the use of
restriction?
Gender, occupation, age

2. The following table shows some baseline characteristics of the
study population according to average childhood body size. The
characteristics listed are considered possible risk factor for
breast cancer.

Average Body Size at ages 5-10 years
Characteristic
1
1.5-2
2.5-3
3.5-4
4.5+
Age (years)
42.5
41.8
42.0
42.3
42.5
Birth weight >=3.9 kg, %
10.0
11.5
13.9
16.2
18.7
Age at menarche
12.8
12.6
12.3
12.1
12.0
Total physical activity at ages 12-17 (MET-hr/wk)
56.3
54.5
51.9
48.4
46.0
Age at first birth (years) (among parous women)
26.3
26.5
26.5
26.5
26.5
Family history of breast cancer, %
9.6
9.7
9.3
9.3
9.5

State which variables are potential confounders and which variables
are not potential confounders according to these data. Give the
rationale for your answer.

3. In the multivariate regression, investigators compare
participants with body size=1 (index group) to participants with
body size ≥ 4.5 (reference group). Table 1 shows the crude and
adjusted risk ratios. The age-adjusted RR controlled only for age.
The MV-adjusted (multivariable-adjusted) RR controlled for many
potential confounders at the same time.

Crude and adjusted RR for the association between adolescent body
size and breast cancer

RR for breast cancer
95% CI
Crude
1.79
1.28-2.19
Age-adjusted
1.64
1.31-2.06
MV-adjusted
1.64
1.29-2.03

3. a. Which of the following is the best interpretation of the
crude RR?
i. In these data, women with adolescent body size≥ 4.5 had 1.79
times the risk of breast cancer compared to those with adolescent
body size=1 over the study period.
ii. In these data, women with adolescent body size=1 had 1.79 times
the risk of breast cancer compared to those with adolescent body
size≥ 4.5 over the study period, adjusting for numerous potential
confounders.
iii. In these data, women with adolescent body size=1 had 1.79
times the crude risk of breast cancer compared to those with
adolescent body size≥ 4.5 over the study period.

3. b. Which of the following is the best interpretation of the
age-adjusted RR?
i. In these data, women with adolescent body size=1 had 1.64 times
the crude risk of breast cancer compared to those with adolescent
body size≥ 4.5 over the study period.
ii. In these data, women with adolescent body size=1 had 1.64 times
the risk of breast cancer compared to those with adolescent body
size≥ 4.5 over the study period, adjusting for age.
iii. In these data, women with adolescent body size≥ 4.5 had 1.64
times the risk of breast cancer compared to those with adolescent
body size=1 over the study period, adjusting for age.
iv. In these data, women with adolescent body size=1 had 1.64 times
the risk of breast cancer compared to those with adolescent body
size≥ 4.5 over the study period, adjusting for numerous potential
confounders.

3. c. State in words your interpretation of the 95% confidence
interval for the MV-adjusted RR.
3. d. What is the magnitude of confounding by this collection of
potential confounders?


3. e. Is this collection of potential confounders actually
confounding the association between adolescent body size and risk
of breast cancer?
a. Yes
b. No
c. Maybe