Matched control case design evaluates the effect of a treatment by comparison of the treated and the non-treated cases in an observational or quasi mode of experiment where treatment is provided randomly. The main advantage of using matched case control design to unmatched control design is it eliminates the effect of confounding variables thereby increasing the efficiency of the study.
The variables used for matched case control design are:
- Age-group ( 18 – 34 years, 35 to 44 years, 45 years or older)
The information about case control design was obtainedby surveying the neighborhoods from fifty meters away from the case’s address. The people were surveyed for the food items they have consumed, according to their questionnaire and any extra food item they consumed to determine the potential source of disease break out.
The main advantage of having a broader definition of the outcome of the cohort study is to investigate the source and causes of post diarrheal hemolytic uremic syndrome. The recipe based restaurant cohort study establishes the link between the possible risk factors and the outcome of the disease.A member of the cohort study was booked as case on basis of laboratory confirmation of shiga toxin producing E.coli O104. In addition, clinical symptom of hemolytic uremic syndrome with diarrhea within two weeks of visiting restaurant K.
Cases where people developed diarrhea only, lacking clinical evidence of the toxin or onset of disease after fourteen days of visiting restaurant k was excluded. As per information received from restaurant’s chef, the likelihood of various food items towards contribution of the outcome carried out by Univariable analysis. Whereas incase control study the study was involved comparing the cases and the control. The recipe based cohort study provides us a broad view of the outcome in association with sources and cohort.
For estimating, the total number of HUS cases caused by consumption of contaminated sprouts in restaurant K during the study period can be calculated by using the formula as stated below:
Total number of main dishes served containing contaminated sprouts = 120
Attack rate among sprout eaters = 30 %
Total no. of HUS cases = 30/100 x 120 = 36
Therefore, the estimated number of HUS cases is 36.
A) Attack rate among sprout eaters:
It represents the percentage of people who have been diagnosed positive for hemolytic uremic syndrome after consuming contaminated sprouts.
It represents the percentage of people fulfilling the criteria as per case definition. Is shows the number of people with symptoms of bloody diarrhea , laboratory confirmed presence of shiga-toxic production of E.coli or hemolytic uremic syndrome with an onset of disease condition within two weeks after visiting restaurant K.
The attack rate among sprout eaters will be bigger if sprouts are reported as the contaminated source.The exposure-outcome association for the case control study is expressedin terms of matched odds ratio (95% CI) and P value. Conditional logistic regression was performed on the variables and two tailed p value used to test the hypothesis. A variable with p- value less than 0.05 is statistically considered.
Among the food item from table 1, statistically we can say sprouts are the primary source of contamination. Sprouts have the lowest p-value among all other food items. The p-value of sprouts 0.04 is less than the threshold value of 0.05; hence, we reject the null hypothesis and acceptthe alternative hypothesis sprouts have effect on the outcome in the German outbreak.
The case controlled study of German outbreak statistically tabulated in table 1, among the five food item listed, the p-value of Sprouts is significant at 95% CI (0.04 < 0.05) with matched odds ratio of 4.35. This means that odds of being infected with HUS increases 4.35 times when sprouts is consumed to the odds of not getting HUS when sprouts is not consumed. The other four-food items p-value is more than the level of significance (> 0.05) and hence statistically it is inferred there is no association between the consumption of these fruits without break of disease.
The 95% CI implies the significance level is 0.05. It means that if the p value associated with a food item is less than significance level or alpha value of 0.05 as in case of sprouts (0.04), the null hypothesis is rejected and the parameter has significant effect on the outcome of disease statistically.
The exposure outcome association for the cohort study expressed in terms of relative risk of infection associated with sprouts and other food items in univariable analysis. The relative risk associated with the food itemmeasured at 95% CI. The level of significance in this study is 0.05. Among all the six food items from table 3, the p-value of Sprouts is significant (0.001 < 0.05). Hence, the null hypothesis is rejected. Statistically it is inferred that Sprouts have significant impact on the epidemic infection.
Calculation of Risk difference as association between consuming sprouts and getting infected
Risk difference = cumulative incidence among exposed subject – cumulative incidence among unexposed subject
Risk difference (RD) = CIe - CIu = (27% of 115 – 0) = 31- 0 = 31.
The risk difference focuses on the absolute effect of the risk factor associated. It measures the excess risk of HUS in those people exposed to sprouts with those who do not. The risk difference associated with consumption of sprouts and getting infecting is 31. This means that out of 115 people exposed to sprouts 27% attack rate recorded than the zero percent attack rate when the subject not exposed to sprouts.