Epidemiology Tutorial #8

Q1 James Lind in 1747 carried out a trial comparing treatments for scurvy, now known to be a consequence of vitamin C deficiency. Provide one strength and one weakness of either the trial design or analysis using the description provided by Gordis (page 198): “I took 12 patients in the scurvy on board the Salisbury at sea. The cases were as similar as I could have them...they lay together in one place and had one diet common to them all. Two of these were ordered a quart of cider per day...Two others took 25 gutts of elixir vitriol...Two others took two spoonfuls of vinegar...Two were put under a course of sea water...Two others had two oranges and one lemon given them each day...Two others took the bigness of nutmeg. The most sudden and visible good effects were perceived from the use of oranges and lemons, one of those who had taken them being at the end of 6 days fit for duty...The other...was appointed nurse to the rest of the sick.” Strength) Designed experiment, tried to select subjects with similar levels of illness, common diet Weakness) Not randomized, no rationale for sample size,No formal statistical analysis, probably not an issue assuming there were no prior patients who spontaneously improved Q2 Which of the following are not suitable scarification variables for random assignments 1. Age 2. Hospital where treated 3. Patient compliance with assi\ged intervention 4. All of the above 5. None of the above C. Since can only be known following random assignments. Variables used for stratified random assignment must be identified immediately prior to random assignment Q3 To be internally valid, the people enrolled in a trial must be representative fo the general population Comment… A sample of clinical trial subjects representative of people in the population with the disease being studied ensures external validity buy tells us noting about internal validity. Consider a clinical trial comparing treatments for women newly diagnosed with cervical cancer. Ideally one would enroll a random sample of such women enhancing external validity. In practice subjects who participate in randomized trials are never a random sample as not all eligible subjects will provide informed consent and so cannot be randomized Q4 Zimmer and Collins (2021) reported results from the Johnson & Johnson vaccine trial. There were approximately 18 Covid cases per 5,000 people among those who received the vaccine. Among the 5,000 people who received placebo there were 63 Covid cases. Argue that the estimated vaccine efficacy is approximately 71%. (Also explain why the authors should have mentioned the length of follow-up) Reference Zimmer C, Collins K. What do vaccine efficacy numbers actually mean? New York Times 2021 (March 3) 2`2` 18 covid cases per 500 who received vaccine 63 covid cases per 500 who received placebo 18 63 1 − ({ 5000 }/{ 5000 }) = 0. 7142857 The length of followup should always be mentioned to allow comparison ith other studies and to enhance interpretation of risk and rate of disease. Q5 Suggest three scenarios when a randomized trial would not be an appropriate study design Randomized controlled trials should not be used when random assignment is 1. Impossible a. Blood types, genetic factor 2. Impractical a. Asking moms to read or bottle feed 3. Unethical a. Asking some people to smoke/ others not to 4. Unnecessary a. Parachutes,insulin for type 1 diabetes