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Case-control studies Practical Exercises Solutions
1. Data from a case control study of periodontal disease and preterm low birth weight delivery conducted by Offenbacher et al. is presented in the table below.
Pre-term Low Birth Weight Delivery
Calculate the Odds Ratio of exposure among cases to that among controls.
Odds ratio (OR) is calculated as the ratio of odds of exposure among cases to that among controls as follows:
OR = A/C = AD = 41 X 9 = 6.71
B/D BC 11 X 5
Interpret your results
Female subjects with periodontal disease were 6.71 times more likely to have a low birth weight infant.
2. In an epidemiological study, 317 female patients suffering from endometrial carcinoma were recruited and questioned regarding their use of oestrogen and the six months prior to diagnosis. The patients were individually matched with 317 other female cancer patients.
The table below shows the results of the study described above.
Oestrogen not Used
What type of epidemiological study is this?
A Case-control study
Compute the Odds Ratio associated with Oestrogen use
Odd Ratio = 152 x 263 / 165 x 54 = 39976 / 8910 = 4.49
It means that those using oestrogen were 4.49 times more likely than those who were not using oestrogen to develop endometrial cancer.
3. In a breastfeeding study, 89 children admitted into a paediatric unit were recruited. Out of these, 47 children had Nasogastric (NG) Tube passed during the course of their admission while 42 did not. At discharge, data was collected on the number of children that had exclusive breastfeeding and those who had part or No breastfeeding out of the two groups. The table below shows the results.
Exclusive Brest feeding
Part/No breast feeding
What is the odds ratio for exclusive breastfeeding at discharge?
Odd Ratio = (27/20) / (10/32) OR (27 x 32) / 20 x 10 = 864 / 200 = 4.32
This can be concluded that breastfeeding at discharge is more likely in NG Tube group.
4. Data in the table below is about information on infant birth weights and mortality among white infants in New York City in 1974.
What is the Odds Ratio for death in the infants in the year 1974?
OR = 618 x 67,093 = 17.6
422 x 5,597
Interpret your results.
The odds ratio illustrates that death is far more likely in the low birth weight group than in the normal birth weight group.
5. Paul et al. (1986) carried out a case control study to investigate the association between the use of oral contraceptives (OCs) and breast cancer in New Zealand. Over a two-year period, cases were identified from the National Cancer Registry while the controls were selected randomly from electoral rolls. Data in the table below were obtained.
Did not use OCs
Estimate the odds ratio for breast cancer in OCs users versus non-users.
Odds ratio = 310 x 189 / 123 x 708 = 0.67.
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