CRITICAL APPRAISAL
   

CRITICAL APPRAISAL:

Here below are 3 examples of a critical appraisal exercise. We will work through them and discuss in class and later if needed on email. Emails to jim@cormack.uct.ac.za

Example 1:

In the Western Cape province, 91 health-workers were identified as having developed hepatitis during 2001. A special study was set up to investigate these cases, wherein a total of 197 health-workers (including the original 91) were enrolled and studied. It was found that overall 122 had experienced a needle stick injury in the period from 1998, and 75 of these were in the original 91.

[20 marks]

a) What type of study is this? [2 marks]
b) What is the appropriate measure of relative risk? [2 marks]
c) Calculate this [4 marks]
d) Explain the meaning of what you have calculated. [2 marks]
e) How would you determine if this was a significant finding? [2 marks]
f) How would you determine if this was a valid finding in this scenario? [5 marks]
g) How would you determine if this was a causal finding in this scenario? [3 marks]
Example 2:

Read and critically appraise the abstract below:

Case-control study of mesothelioma in South Africa.
Rees D, Myers JE, Goodman K, Fourie E, Blignaut C, Chapman R, Bachmann MO.

BACKGROUND: South Africa has, uniquely, mined, transported, and used crocidolite, amosite, and chrysotile. A multicenter case-control study was done in South Africa to examine the details of asbestos exposure in cases and controls, and to calculate relative risks for level of certainty of asbestos exposure, nature of exposure (e.g., environmental, occupational) and fiber type.

METHODS: Cases and controls (one cancer and one medical per case) were collected by six study centers from referral hospitals, and exposure information was collected by interviewing cases and controls in life.

RESULTS: One hundred and twenty-three cases were accepted into the study. None had purely chrysotile exposure. Twenty-three cases had mined Cape crocidolite; three had mined amosite; and three Transvaal crocidolite plus amosite. A minimum of 22 of the cases had exclusively environmental exposure, 20 were from the NW Cape crocidolite mining area. The relative risks associated with environmental exposure in the NW Cape (crocidolite) were larger than for environmental exposure in the NE Transvaal (amosite and crocidolite): 21.9 vs. 7.1 and 50.9 vs. 12.0 for the cancer control and medical control datasets, respectively.

CONCLUSIONS: The results confirm the importance of environmental exposure in the Cape crocidolite mining area, the relative paucity of cases linked to amosite, the rarity of chrysotile cases and are consistent with a fiber gradient in mesotheliomagenic potential for South African asbestos with crocidolite > amosite > chrysotile.

  1. What was the research question?
  2. What was the study design?
  3. Why was this study design appropriate?
  4. What was the study population?
  5. Describe the comparison groups.
  6. Were there any sources of information or selection bias?
  7. How did the investigators control for confounding?
  8. What are the main conclusions of the study?
  9. Were these conclusions justified?
Example 3:

You read the abstract of a study done to determine whether exposure to video display terminals (VDTs, i.e. computer monitors) is harmful to pregnancy. This case-control study of the association between time spent per week working at a personal computer during pregnancy (over the last year) and spontaneous abortion found the following:

Time spent
(hrs per week)		 Relative risk of abortion
(each stratum compared to lowest)		 P-value
1 - 10 1.00 -
11 - 20 1.70 0.15
> 20 2.00 0.03

Answer the following questions, and indicate where necessary what other information (not given in the question) you would need to answer the question. Simple yes or no answers are not sufficient - a few sentences motivating each answer are required.

  1. What population was studied here?
  2. Describe the association(s) found in this study to a layperson, without using technical terms.
  3. Is the association statistically significant? Justify your answer, including your understanding of what is meant by "statistical significance".
  4. What does statistical significance tell you about the validity of the study finding? Justify your answer, including your understanding of what is meant by "validity" of a study.
  5. Could there be confounding and bias in this study? Justify your answer, including your understanding of what is meant by "confounding".
  6. Do you think the association is causal? What criteria would you use in this case to help you decide whether the association is causal?
  7. If the association were causal, what criteria would you use to determine whether it is of public health importance?