Answer Happy

Assessing the Role of Bias
The role of diet in promoting or preventing disease is a focus of
nutritional epidemiology. In recent years, the field has embraced
the idea that dietary patterns, as opposed to a single food or
nutrient, are likely to have a bigger impact on health and disease
prevention. The Mediterranean diet is characterized by the
traditional food patterns of Mediterranean countries. The
Mediterranean diet has consistently been shown to reduce the risk
of all-cause mortality, cardiovascular disease mortality, and
cancer mortality in several large prospective cohort studies.
Suppose that we work for the American Cancer Society, which would
like to update its “Guidelines on Nutrition and Physical Activity
for Cancer Prevention.” Guidelines are based on current evidence,
and their purpose is to provide recommendations for the general
public.
We are tasked with critically evaluating one recent study that
examined the association between Mediterranean diet and risk of
ovarian cancer in the Nurses’ Health Study, a prospective cohort
study with 121,700 female nurses (Xie et al, Journal of Ovarian
Research (2014) 7:112).
In this study, dietary information was measured prospectively every
2 to 4 years by a self-administered semi-quantitative, 131-item
food frequency questionnaire. For each food, a portion size was
given, and women were asked to choose from 9 intake frequencies
from never to >=6 servings per day averaging over the prior
year’s intake. A Mediterranean Diet Score was calculated that
considered consumption of certain fatty acids, legumes, cereals,
fruits, nuts, vegetables, meat, dairy, and alcohol and ranged from
0 to 10.
The following excerpts are taken from the paper.

1. “The validity of our food frequency questionnaire was evaluated
by comparison with food diaries in 192 women.” By comparing two
ways of collecting the same type of data, which type of bias are
the study investigators trying to limit?
2. If the correlation between the food frequency questionnaire
and food diaries is poor, is it more likely or less likely that
this type of bias is present?

3. Ovarian cancers were identified either by self-reports or data
from the National Death Index. Women or their next-of-kin were
asked for permission to obtain and review pathology reports to
confirm the diagnosis. Which type of bias are the study
investigators trying to limit by confirming the ovarian cancer
diagnosis?


4. “The follow-up rate was 97%.” Which type of bias are the
investigators trying to assess?


5. Based on the follow-up rate, is this type of bias likely?

6. Is the “Healthy Worker Effect” a concern in this study?

7. “Participants who left more than 70 items blank on the diet
questionnaire were excluded from the analysis.” Which types of bias
were the study investigators trying to limit?

8. “Participants who left more than 70 items blank on the diet
questionnaire were excluded from the analysis.” Did excluding these
women create selection bias?
9. “All of this [dietary) information was collected prospectively...” What type of bias was avoided with prospective dietary data collection? Hazard ratios for the association between Mediterranean Diet Score and ovarian cancer Mediterranean Diet Score <2.6 2.7-3.5 3.6-4.5 4.6-5.5 >5.5 P trend N cases 134 119 148 136 159 Person-years 290,089 247,467 329,112 293,048 309,867 1.0 (ref) 0.91 0.85 0.86 0.93 0.52 Age-adjusted hazard ratio 1.0 (ref) 0.90 0.83 0.85 0.91 0.44 Multivariable- adjusted hazard ratio