Physical activity is the centerpiece of many strategies to prevent and reduce chronic disease and improve health.
5. 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. Additional Information that may be necessary: The main exposure is body size. The main outcome is breast cancer. A crude model is a regression model that only has the main exposure variable and the main outcome variable in the model. S_othe crude model looks only at the relationship between the exposure and the outcome. fiadjusted model means a model that has the main exposure variable, the main outcome variable, and at least one other variable (the potential confounder} in the modjel. Sometimes adjusted models have the main exposure variable. the main outcome variable. and several other variables (several potential confounders) in the model. Note that the crude model and any adjusted models are still looking at the same research question: Is the exposure associated with the outcome? However. adjusted models are controlling for one or more potential confounding variables while answering the research question to see if the estimate (the risk ratio in this example) changes when the confounder(s) isr’are controlled for. Generally, we say that a change-in-estimate comparing the crude to the adjusted estimate of 10% or greater indicates confounding is present although this 10% cut-point is arbitrary. Table 1: Crude and adjusted risk ratios (RR) for the association between adolescent body size and breast cancer RR for breast cancer 95% Cl Crude 1.79 1.28-2.19 Age-adjusted 1 .64 1.31-2.06 MV-a dj usted 1 .64 ‘l .2 9-2 .03
9—15 points} What is the magnitude of confounding by age? You should calculate the change-in- estimate for the crude risk ratio compared to the age-adjusted risk ratio to answer this question. Change in estimate = L_(_5 points) Is age actually confounding the association between adolescent bodyr size and the risk of breast cancer? Justify your answer.
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