epi quiz 2- Study Design
In which scenario would it be best to select a case-control study design over a retrospective cohort study design? (select one best answer)
The outcome being studied is rare A case control study is efficient for studying rare diseases because individuals are select based on disease status. In retrospective cohort studies, individuals are selected from a general cohort or based on exposure before occurrence of outcomes, thus very few participants would be expected to develop a rare exposure (resulting in insufficient power to study the exposure-disease relationship). Case-control studies can also be less expensive and take less time to complete than cohort studies.
Which of the following statements are true about matching of cases and controls in a case-control study? (select one best answer)
A) Each case is matched to one or more controls to improve efficiency of the study B) Matching seeks to provide a similar distribution important study variables between cases and controls (i.e., sex, age-grouping) D) Matching can be used to ensure an adequate number of cases and controls in exposure categories where a disproportionate case/control ratio is expected F) A, B and D are true All of the statements are true about matching in a case-control study except option C) "The matching factors can then be studied as determinants of the relationship between disease and exposure in the analysis". When you match on a factor in a case-control study, you cannot study it as a determinant.
What are essential considerations when selecting controls for a case-control study? (select one best answer)
A. Controls should be representative of the population that gave rise to the cases D. Controls should not have the outcome at time of enrollment A and D Instructor Explanation: There are a variety of techniques for selecting controls for a case-control study. The only two essential considerations are (1) that the controls are representative to the population that gave rise to the cases, and (2) that the controls do not have the outcome of interest at the time of enrollment. Matching on some characteristics is optional, and may be desirable under certain conditions, but is not essential.
A case-control study is characterized by all of the following except (select one best answer):
A. It is relatively inexpensive compared to a cohort study. B. Persons with the disease (cases) and compared to persons without the disease (controls). ***C. Incidence rates can be computed directly.*** D. Assessment of past exposure may be biased. Instructor Explanation: Options A, B and D are all characteristics of a case-control study. Option C is not. You cannot directly calculate incidence rates in a case control study because you enroll based on disease status and retrospectively determine exposure (thus you cannot observe incident events).
Read the journal excerpt below: BACKGROUND: Rosacea is a common chronic inflammatory dermatosis of unclear origin. It has been associated with systemic comorbidities, but methodical studies addressing this association are lacking. OBJECTIVE: We evaluated: (1) the association between rosacea and systemic comorbidities; and (2) if the severity of rosacea is impacted by comorbidities. METHODS: Patients with rosacea were matched (1:1) to rosacea-free control subjects by age, sex, and race. Systemic comorbidities were determined for each case and control by reviewing electronic medical records. RESULTS: Among 130 participants (65 patients/65 control subjects), we observed a significant association between rosacea and allergies (airborne, food), respiratory diseases, gastroesophageal reflux disease, other gastrointestinal diseases, hypertension, metabolic and urogenital diseases, and female hormone imbalance. Compared with mild rosacea, moderate to severe rosacea was significantly associated with hyperlipidemia, hypertension, metabolic diseases, cardiovascular diseases, and gastroesophageal reflux disease. CONCLUSIONS: Rosacea is associated with numerous systemic comorbid diseases in a skin severity-dependent manner. Physicians should be aware of these associations to provide comprehensive care to patients with rosacea, especially to those with more severe disease. What study design was used? (Choose one best answer)
Case-control This is a case-control study. People with rosacea were selected to be in the study and matched with control subjects. The investigators compared those who had rosacea to those who did not have rosacea to see if those who had rosacea had higher odds of having comorbidities.
Read the journal excerpt below: Exposure to light at night suppresses the physiologic production of melatonin, a hormone that has antiproliferative effects on intestinal cancers. Although observational studies have associated night-shift work with an increased risk of breast cancer, the effect of night-shift work on the risk of other cancers is not known. We prospectively examined the relationship between working rotating night shifts and the risk of colorectal cancers among female participants in the Nurses' Health Study. We documented 602 incident cases of colorectal cancer among 78 586 women who were followed up from 1988 through 1998. Compared with women who never worked rotating night shifts, women who worked 15 years or more on rotating night shifts were 1.35 times more likely to develop colorectal cancer (95% CI = 1.03 to 1.77), adjusting for confounders. These data suggest that working a rotating night shift at least three nights per month for 15 or more years may increase the risk of colorectal cancer in women. What study design was used? (select one best answer)
Cohort Instructor Explanation: You can tell this is a cohort study because the women were put into groups based on their exposure status (never worked night shifts vs 15+ years of working night shifts) and followed to see if they developed the disease (colorectal cancer).
Read the journal excerpt below: An investigator is interested in whether children who live in houses with cats or dogs have lower prevalence of allergic disease than children who live in houses without cats or dogs. To answer this question, she designs a survey and randomly samples 1,100 households with children under 13 years old in the Denver metropolitan area. At each household, she asks the parent to report whether they currently have cats or dogs in the house, and whether their child suffers from asthma, eczema, allergic rhinitis, or other allergic diseases. She then calculates a prevalence ratio for allergic disease among children in households with pets as compared to children in households without pets. What study design was used? (Select one best answer)
Cross-sectional A random sample is selected from a population of interest, without regard to disease or exposure. A survey was then used to ascertain both the exposure (cats or dogs in the household) and the outcome (any prevalent allergic disease in the child), at the same point in time.
In which of the following epidemiological study designs is the unit of investigation populations or groups of people rather than individuals (select one best answer)?
Ecologic study When the unit of analysis is a population or group, this is an ecologic study. They are similar to cross-sectional studies in that they compare the prevalence of exposure and outcome at one point in time. However, the unit of analysis in a cross-sectional study is the individual.
Read the journal excerpt below: According to de Catanzaro's (1981) evolutionary theory of human suicide, threshold intelligence is necessary for suicidality. Thus there is interest in determining if Intelligence and suicide mortality are positively correlated. This theory was investigated in a study that included 85 countries. Average national IQ was found to be significantly positively related to the national male and female suicide rate. The relationship was not attenuated by controlling for the countries' per capita Gross Domestic Product or the type of national IQ estimation. What study design was used? (select one best answer)
Ecological This is an ecological study design. The key detail here is that the investigators looked at National IQ levels and suicide rates - they did not look at individual-level data.
Which of the following is NOT true of cohort studies? (select one best answer)
Incidence rate may be calculated directly ***They are relatively inexpensive compared to case-control studies*** Recall bias is minimized compared to a case-control study because exposure is measured at the start of the study Multiple disease outcomes can be studied Allow for the ability to measure changes in risk factors over time Instructor Explanation: All of the above are characteristics are characteristics of the cohort study design except the statement about the expense relative to case-control studies. Cohort studies tend to be more expensive than case-control because of the expense of collecting data over an extended period of time.
Read the journal excerpt below: In 2000, 201,743 women between the ages of 45 and 65 were recruited to participate in a study that assessed whether greater access to preventive care was associated with earlier detection of cervical cancer (cancer diagnosed at stage I). At the start of the study, medical records were obtained documenting the number of pap smears each participant had received in the 10 years before their enrollment in the study. After enrollment, women were followed for 10 years to determine if they had been diagnosed with cervical cancer and at what stage their cancer was diagnosed (stage I, II, etc.). By 2010, 87 cases of stage I cervical cancer and 24 cases of stage II cervical cancer were reported. The researchers found that there was a lower incidence of stage II cervical cancer among women who had received a greater number of gynecological exams where a pap smear was done. What study design was used? (Choose the one best answer)
Retrospective cohort Instructor Explanation: This is a retrospective cohort because information on exposure status (number of pap smears) has already been documented and is available for ascertainment at the time of study enrollment (T0), while the outcome is assessed prospectively (10 years later) after enrollment (T0).
Why is a cohort design more efficient than a case-control design to study a rare exposure? (Select one best answer)
Subjects can be selected based on their exposure status. A cohort study is efficient for studying rare exposures because it can be structured to select people based on exposure status. The case-control study design selects participants based on their case/outcome (disease) status and would likely not capture many individuals that experienced a rare exposure.
