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Old 06-25-2011
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Stats multi-national study--what is the design?

An epidemiologist is attempting to determine if there is a relationship between alcohol intake and pancreatic cancer. Incidence rates of pancreatic cancer across many different countries are compared against the per capita consumption of alcohol in each of these countries. What study design is this epidemiologist using?

Answer Choices
A. Case-control study
B. Cohort study
C. Cross-sectional study
D. Ecologic study
E. Randomized control trial
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Old 06-25-2011
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Default Ecologic study.

The answer is D.

An ecological study is an epidemiological study in which the unit of analysis is a population rather than an individual. For instance, an ecological study may look at the association between smoking and lung cancer deaths in different countries. An ecological study is normally regarded as inferior to non-ecological designs such as cohort and case-control studies because it is susceptible to the ecological fallacy.
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Option D (Ecologic study) is correct. Ecologic studies use group data to determine associations between two variables. In this case, per capita alcohol use is compared against incidence rates of pancreatic cancer. These data are group data and do not necessarily link high individual alcohol use to pancreatic cancer. This is one of the major weaknesses of ecologic studies.

Option A (Case-control study) is incorrect. Case-control studies are used to help determine the etiology of diseases, especially rare diseases. Case-control studies start with the investigators knowing who has the disease of interest. The cases are those with the disease and the controls are those without. The study then attempts to characterize exposure to a particular agent.

Option B (Cohort study) is incorrect. A cohort study starts with individuals who are either exposed or not exposed to a particular agent. These two groups are then followed for a period of time to determine if disease develops. Cohort studies can be prospective or retrospective.

Option C (Cross-sectional study) is incorrect. Also known as a prevalence study, this type of study design is used to determine disease and exposure at the very same moment. This type of study is useful for determining risk factors.

Option E (Randomized control trial) is incorrect. A randomized control trial is used to determine the efficacy of therapy. A groups with a particular indication for therapy is randomized and then split into two cohorts: one that will receive the therapy and one that will receive a placebo. They are then prospectively followed to determine specific outcomes.

High-yield Hit 1
Conceptually, a two-step process is followed in carrying out studies and evaluating evidence. However, in practice, this process often becomes interactive and deviates from a fixed sequence:
We determine whether there is an association between an exposure or characteristic and the risk of a disease. To do so, we use:
Studies of group characteristics: ecologic studies
Studies of individual characteristics: case-control and cohort studies
If an association is demonstrated, we determine whether the observed association is likely to be a causal one.

From Epidemiology 3E by Gordis
High-yield Hit 2
Ecologic Studies
The first approach in determining whether an association exists might be to conduct studies of group characteristics, called ecologic studies. Figure 14-4 shows the relationship between breast cancer incidence and average dietary fat consumption in each country.2 In this figure, each dot represents a different country.

Figure 14-4 Correlation between dietary fat intake and breast cancer by country. (From Prentice RL, Kakar F, Hursting S, et al: Aspects of the rationale for the Women's Health Trial. J Natl Cancer Inst 80:802-814, 1988.)
The higher the average dietary fat consumption for a country, the higher breast cancer incidence for that country generally is. We might therefore be tempted to conclude that dietary fat may be a causal factor for breast cancer. What is the problem with drawing such a conclusion from this type of study? Consider Switzerland, for example, which has a high breast cancer incidence and a high average consumption of dietary fat. The problem is that we do not know whether the individuals in whom breast cancer developed in that country actually had high dietary fat intake. All we have are average values of dietary fat consumption for each country and the breast cancer incidence for each country. In fact, one might argue that given the same overall picture, it is conceivable that those who developed breast cancer ate very little dietary fat. Figure 14-4 alone does not reveal whether this might be true; in effect, individuals in each country are characterized by the average figure for that country. No account is taken of variability between individuals in that country in regard to dietary fat consumption. This problem is called the ecologic fallacy-we may be ascribing to members of a group, characteristics that they in fact do not possess as individuals. This problem arises in an ecologic study because we only have data for groups; we do not have exposure and outcome data for each individual in the population.
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Biostatistics-Epidemiology, Step-2-Questions

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