Question

1. How are cases and controls selected? 2. What is relative risk? Exposure? 3. What is a confounding variable? 4. What is bias in a research study? What are some of the sources of bias in epidemiologic studies? 5. What is a cross-sectional study?

Answer 1

Case- Control study design is atype of observational study that compares two groups of people : those with the disease or condition under study (cases) and very similar group of people who donot have the disease or condition(controls).

1.In case controls studies, cases and controls are selected from an existing cohort. so that the two groups are mached on all characteristics that could possibly infiuence the outcome expect the exposure status. In a case- control study, participants are selected for the study based on their outcome status. Thus some participants have the outcome of interest(cases), whereas others do not have the outcome of interest(controls). The investigator then assesses the exposure in both these groups.

2. Relative risk is a ratio of the probability of an event occuring in the exposed group versus the probability of the event occuring in the non-exposed group.

Relative risk= probability of event in exposed group/ probability of event in non-exposed group.

Exposure means any factor that may be associated with an outcome of interest. The exposure refers to any characteristics that may explain or predict the presence of a study outcome.

3. Confounding variable is an outside influence that changes the effect of a dependant and independant variable. Simply, a cofounding variable is an extra variable entered into the equation that was not accounted for. This extraneous influence is used to influence the outcome of an experimental design. Confounding variables are variables that the researcher failed to control, or eliminate, damaging the internal validity of an experiment. It can ruin the experiment and produce useless results.

4. Bias is any trend or deviation from the truth in data collection, analysis, interpretation and publication which can cause false conclusions. It can occur intentionally or unintentionally. Bias technically means a systematic error, where a particular research finding deviates from a true finding and creates errors in research.

Common sources of bias in epidemiological studies are

• Observer bias - Occurs when there are systematic differences in the way information is collected for the group being studied.It occurs as a result of investigator's prior knowledge of hypothesis under investigation or knowledge of the individual's exposure or disease status.
• Selection bias- Occurs when two groups being compared differ systematically. There is difference in the characteristics between those who are selected for a study and those who are not selected.
• Sampling bias -Occurs when the process of sampling actually introduces an inherent bias into study.
• Procedural bias- Occurs where an unfair amount of pressure applied to subject forcing them to complete their responses quickly.
• Response bias- Occur wher the subject consciously or sub consciously gives response that they think that the interviewer wants to hear.

5. A Cross- Sectional study involves looking at data from a population at one specific point in time.

The participants in this type of study are selected based on particular variables of interest. Cross sectional studies are observational in nature. Characteristics includes

- Study takes place at single point of time.

-It does not involve manipulating variables.

- It allows researcher to look at once (age, income, gender)

-Often used to look at the prevailing characteristics in given population.