Case-Control Study

Data Collection

With the design of the case-control study complete, you now begin planning the protocol for data collection.

4. What is the best source for ascertaining cases? [Aschengrau & Seage, pp. 236-239]

  1. Identification of cases of Susser Syndrome from hospital charts of the local hospital. Diagnoses are based on clinical assessment and are supported by brain imaging results.
  2. Identification of subjects complaining of neurological symptoms from the records of the Superfit Center's staff nurse
  3. Identification of cases from the records of a prominent Epiville neurologist in private practice
Answer (a) — correct: Recall from the Epiville Department of Health website that a diagnosis of Susser Syndrome requires a neurological consult. In addition, according to the EDOH website, the neurological symptoms of Susser Syndrome are so severe that all cases will end up at the local hospital. Therefore, a review of the hospital charts should offer an excellent source of valid data on every case in Epiville.
Answer (b) — incorrect: We might miss individuals with the Syndrome who never saw the staff nurse and might erroneously include those whose symptoms are not due to Susser Syndrome.
Answer (c) — incorrect: While some cases will see a neurologist for their symptoms, available information on the Susser Syndrome (Epiville Department of Health website) indicates that some will be very ill and will be admitted directly to the hospital emergency room before having visited a specialist. Thus, we might miss persons with the Syndrome, or we might erroneously include persons whose symptoms are not Susser Syndrome.

5. What is the best method of ascertaining the cases? [Aschengrau & Seage, pp. 236-239]

  1. Look through the records of the local hospital dated in the last two years to identify those with the disorder.
  2. Look through the records of the local hospital dated in the last two years to identify those with the disorder and see if there is information in the chart about their membership in the Superfit Fitness Center
  3. Link the computer database of the Membership Directory of the Superfit Fitness Center with the discharge database of the local hospital
Answer (a) — incorrect: Since we defined our source population as those who attend the Superfit Fitness Center, we need to limit our cases to those arising from this population.
Answer (b) — incorrect: Although logical, this is not the best method of ascertaining cases since one's gym membership is not normally part of a medical workup. In addition, cases that terminated their club membership as a result of their illness would be missed.
Answer (c) — correct: Modern computer linkage techniques allow for correct identification of approximately 95% of people based on their name and birth date. Because the local hospital treats all potential cases, the only cases you are likely to miss are those who moved out of the area before treatment or those patients discharged after the study end date. Because of the setup, all cases would come from the population attending the Superfit Fitness Center.

6. What is the best way to accrue the controls? [Aschengrau & Seage, pp. 239-243]

  1. Cross-reference the Superfit Fitness Center 's Membership directory with the local hospital's discharge database taking patients treated there for diseases other than the Susser Syndrome.
  2. Ask the Membership Services of the Superfit Center to provide you with the names of new members who have enrolled in the last year and are active members in good standing.
  3. Take a random sample of people from the Membership Directory of the Superfit Fitness Center and determine their eligibility criteria. Those eligible will be asked to participate in the study by phone. If they agree to participate, an interview will be scheduled
Answer (a) — incorrect: This method would be unlikely to produce enough controls since the majority of Superfit members would not be admitted to the hospital, because they are otherwise a young and healthy population. In addition, inclusion of controls with other diseases may introduce bias because other diseases may also be related to the exposure of interest and thus controls would not be sampled independently of exposure.
Answer (b) — incorrect: Given Susser Syndrome's induction time of 6 months, some of the new members will not have the opportunity to be classified as "exposed," meaning that this sampling technique selects controls dependent on exposure status. In addition, this is not a random sample as new members may not be representative of the entire population of those attending the Fitness Center.
Answer (c) — correct: Because of the membership at the club, these controls will be representative of the underlying source population from which the cases emerged. Also, you must make sure that they do not have the disease.

Now you must decide how you will assess exposure to Quench-It and Endurobrick in your case-control study.

7. Given the study design, what is the best way to define the exposure variable?

  1. Directly observe the consumption of EnduroBrick or Quench-It at the Superfit Fitness Center
  2. Question all study subjects as to their own assessment of EnduroBrick and Quench-It consumption using a standardized survey tool.
  3. Use the Superfit Fitness Center credit card billing system to identify exposure to EnduroBrick and Quench-It in the members.
Answer (a) — incorrect: This is a case-control study in which the exposure has already occurred. Study participants are enrolled based on their disease status, after which exposure is compared between those with the disease and those without the disease.
Answer (b) — incorrect: This is commonly employed and it is probably the fastest and cheapest method; however, it is problematic here as individuals may have difficulty recalling the amounts consumed over a 2- year period. In addition, now that the news is reporting a connection between Susser Syndrome and Superfit Fitness Center, the interviewee's responses may be differentially affected by their concerns over what they believe may be the cause of the condition
Answer (c) — correct: This will provide the most accurate assessment of exposure since it does not depend on the ability of the subject to recall their exposure. Furthermore, we know EnduroBrick and Quench-It are not available in retail stores, so there is no possibility of subjects consuming more than is recorded by the fitness center. (It is not perfect, however, as purchasing EnduroBrick and Quench-It is not a guarantee that the individual actually consumed it!!!)

Having received the necessary IRB approval and addressed various administrative details , the study may now commence. The data begin to file back to the Department of Health and must now be carefully entered into the computer database. Once all of the data are entered, you can proceed to the analysis stage where the associations proposed in your two hypotheses are characterized and tested.