How common is the disease? How common is the exposure? Do you have access to a study population?
All Modules Cohort Studies and Case-Control Studies The cohort study design identifies a people exposed to a particular factor and a comparison group that was not exposed to that factor and measures and compares the incidence of disease in the two groups.
A higher incidence of disease in the exposed group suggests an association between that factor and the disease outcome. This study design is generally a good choice when dealing with an outbreak in a relatively small, well-defined source population, particularly if the disease being studied was fairly frequent.
The case-control design uses a different sampling strategy in which the investigators identify a group of individuals who had developed the disease the cases and a comparison of individuals who did not have the disease of interest. The cases and controls are then compared with respect to the frequency of one or more past exposures.
If the cases have a substantially higher odds of exposure to a particular factor compared to the control subjects, it suggests an association. This strategy Case controlled study vs cohort a better choice when the source population is large and ill-defined, and it is particularly useful when the disease outcome was uncommon.
Examples of two real outbreaks will be used to illustrate these differences in sampling strategy. Health officials noted that an unusually large number of cases had been reported during a span of several days.
The table below summarizes some of the salient facts about Salmonella infections. Descriptive epidemiology was conducted, and hypothesis-generating interviews indicated that all of the disease people had attended a parent-teacher luncheon at a local school.
In fact, it was a potluck luncheon, and the attendees each brought a dish that they had either prepared at home or purchased.
The descriptive epidemiology convincingly indicated that the outbreak originated at the luncheon, but which specific dish was responsible?
The investigators needed to establish which dish was responsible in order to clearly establish the source and to ensure that appropriate control measures were undertaken.
Diarrhea, fever, abdominal cramps, vomiting. Paratyphi produce typhoid with insidious onset characterized by fever, headache, constipation, malaise, chills, myalgia; diarrhea is uncommon and vomiting is usually not severe. Contaminated eggs, poultry, unpasteurized milk or juice, cheese, contaminated raw fruits and vegetables alfalfa sprouts, melons.
Typhi epidemics are often related to fecal contamination of water supplies or street vended food. Other sources include pet rodents hamsters, mice, and rats, or their bedding and reptiles and amphibians e.
Of these, 45 attendees agreed to complete a questionnaire regarding the foods that they had eaten at the luncheon. Since they had a relatively small, discrete cohort and a fairly high incidence of disease, a cohort design was a logical choice.
For each dish served at the luncheon the investigators compared the incidence of Salmonellosis between those who ate a particular dish the exposed group and those who had not eaten that dish the non-exposed comparison group. For each dish they constructed a contingency table to summarize the result from the survey.
For example, the table below summarizes the findings from the survey regarding the incidence of disease in those who ate the cheese appetizer compared to those who did not eat it.
These results indicate that 23 attendees recalled eating the cheese appetizer, and 16 of them subsequently developed Salmonellosis, i.
When comparing the incidence of disease in an exposed group and an unexposed group, the magnitude of association is often summarized by computing a risk ratio, as follows. In order to complete the analysis, the investigators performed these computations for each of the dishes served at the luncheon.
The table below summarizes all of the findings. However, the overall sample was very small, and some of the dishes had very few takers, such as the potato salad.
It is not surprising then that the risk ratios column "RR" vary above and below a value of 1 as a result of random error i. Conversely, the true value is unlikely to lie outside this range.
The confidence interval also provides a measure of the precision of the estimated risk ratio. The last three columns, then, help us put all of this into perspective.
Most of the risk ratios RR are somewhat above or below a value of 1. However, the risk ratio for exposure to manicotti was Finally, the p value was less than 0. It would, therefore, be reasonable to conclude that the manicotti was the source of the Salmonella outbreak.
For more information about cohort studies, risk ratios, confidence intervals, and p values, please consult the following modules:Cohort Studies and Case-Control Studies.
The cohort study design identifies a people exposed to a particular factor and a comparison group that was not exposed to that factor and measures and compares the incidence of disease in the two groups.
by Annette Gerritsen, Ph.D. Two designs commonly used in epidemiology are the cohort and case-control studies. Both study causal relationships between a risk . A cohort study is a particular form of longitudinal study that sample a cohort The study is controlled by including other common characteristics of the cohort in the statistical analysis.
Both exposure/treatment and control variables are measured at baseline. Nested case-control study An example of a. Key Difference – Cohort vs Case-Control Study Cohort and case-control study are two of the designs used in research between which some difference can be identified.
A researcher who is about to conduct a study on a specific field usually has research objectives and questions. Key Difference – Cohort vs Case-Control Study Cohort and case-control study are two of the designs used in research between which some difference can be identified.
A researcher who is about to conduct a study on a specific field usually has research objectives and questions. Introduction. Case-control and cohort studies are observational studies that lie near the middle of the hierarchy of regardbouddhiste.com types of studies, along with randomised controlled trials, constitute analytical studies, whereas case reports and case series define descriptive studies (1).