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Epid 2200 tutorial 8
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Epid 2200 tutorial 8 with question and answer
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Q1James 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
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