Question :

1. You heard of two wedding parties last weekend. Both parties were held at the same hotel but four hours apart. We can reasonably assume that the food was prepared by the same personnel, both menus were similar. There were 325 guests at the earlier party. 200 guests ate Cole slaw, 40 complained of stomach cramps and diarrhea. Similar symptoms were reported by 25 individuals among those who did not eat Cole slaw.

The second party, held later, had a total of 522 guests; 260 ate Cole slaw (100 people who ate Cole slaw and 162 who did not eat) reported same symptoms as observed from earlier party.

Is Cole Slaw the food that made the guests sick? How do you know?

2. In an area of Washington the occurrence of multiple sclerosis (MS) was investigated among the native Caucasians population (679,478 individuals) and among those of Japanese origin (16,122 individuals). At the time of the investigation it was found that for the two groups there were 496 cases of MS, half of which were Caucasians. During the 12 months of study, the population had increased (13,789 Caucasians were added, and so were 21254 Japanese), due to flight from Mount St Helen volcano eruption. All newcomers were healthy when they arrived. During the 12 months of study 187 new cases were discovered of which 105 were also Caucasians. At the end of the 12 months of study 287 new cases were discovered of which 110 were Caucasians.

Which measure(s) of disease occurrence can be calculated? Calculate it (them) for each group

3. You wish to diagnose early occurrence of uterine carcinoma in women age 25 -70. You gather a population of 655,999 women age 24-75. At the time of examination, you discover that three percent (3%) of them have been treated for this condition. After 12 months of observation, you detect an additional 3% of cases. What measurements of disease frequency can you calculate?

4. Among those newly admitted to a psychiatric treatment center there were confirmed carriers of hepatitis B who were to be placed in one ward but not in others. To investigate to what extent this will affect the workers at the treatment center; you decided to test all workers for the presence of hepatitis B with a serological test (detection of HBs antigen by ELISA). Of 97 people working on the ward where carriers will be housed, 16 had markers for hepatitis B; of 72 people working on the other wards, 9 had these markers. Which measure of occurrence of markers can be calculated?

5. In a suburb of Stockholm there were 21 cases of injury due to moped accidents in one year, while in a city parish with the same average population size (80,000 people) only 9 such injuries occurred (See table).

No. of injuries and person years by age and area

Age Number of injuries Number of person years
(years) Suburb City Suburb city
15 19 20 7 4,000 1,000
20+ 1 2 76,000 79,000
Total 21 9 80,000 80,000

a) Calculate the incidence rate for the suburb and the city parish, respectively, without taking the age distribution of the two populations into account.

b) Calculate the specific incidence rate for each age group in the suburb and the city parish, respectively.

6. You wish to know whether there is some truth to generally held belief that exposure to radon gas might be associated with increased lung cancer in Chester County, PA. You went to Chester County Hospital and looked at the records of patients who were admitted for various lung cancers during the period of twenty years (1970 to 1990).
Finding: You studied 911 charts
544 had been exposed to radon
410 lung cancers (177 were not exposed to radon)

Calculate the prevalence of cancer in those exposed to radon and in those who did not have exposure to radon.

Calculate the prevalence of exposure in those who had cancer and in those who did not have cancer.

Do these results suggest that a better and more controlled study should be conducted on this subject? Explain your reason.

7. From the data in Table below, Calculate
a. Relative risk of smokers versus nonsmokers
b. Attributable risk for smokers
Table Death Rates from Lung Cancer in People Aged 35 or Older
Death Rates, from Lung Cancer, per 1,000 People A 35 or Older, per year
Non smokers 0.07
Smokers 0.96

8. In a certain noisy factory, workers are provided with earplugs and expected to wear them. An industrial hygienist inspecting the plant found that 458 of the 1735 workers in the factory were not wearing their earplugs because they thought they were not cool. When all workers were given a hearing test, it was found that 97 of the earplug wearers and 298 of the nonwearers had developed significant hearing loss. All had normal hearing at their preemployment examinations 6 years earlier, when the plant first opened.

a. What was the risk of hearing loss attributable to not wearing earplugs?

b. What was the relative risk in nonwearers as compared to wearers?

9. A simple and inexpensive screening method has been developed to identify individuals with HIV infection at routine medical check-ups. To study the sensitivity and specificity, the method was tested on 1860 persons who underwent a simultaneous and thorough clinical examination that was considered to give an accurate diagnosis.

The results were as follows:

The clinical test identified 1160 individuals as having the disease, whereas the screening method identified 480 as being positive for the disease. The screening method correctly identified 160 among those with clinical disease.

Calculate the sensitivity of the screening method.

Calculate the specificity of the screening method.

10. In a mass screening aimed at early diagnosis of a HIV infection, a screening instrument described in question # 9 is used. Of 2500 individuals attending the screening, 384 have the disease. What proportion of those who will screen positive will be diseased?