MadSci Network: Medicine

Re: what is a 'randomized','controlled','blind/double blind'study?

Date: Tue Oct 9 15:46:52 2001
Posted By: Kevin Caldwell, Faculty, Neurosciences, University of New Mexico
Area of science: Medicine
ID: 1000870017.Me

The following information is quoted from "Basic & Clinical Biostatistics", 
3rd edition, by Beth Dawson and Robert G. Trapp, McGraw-Hill, 2001  This is 
an excellent book that you may be able to find in a medical library.

[My additions are in brackets.]

A simple random sample is one in which every subject has an equal 
probability of being selected for the study.  The recommended way to select 
a simple random sample is to use a table of random numbers or a 
computer-generated list of random numbers.  (pp. 69-70).

[There are types of random sampling- for example, a systematic random 
sample, a stratified random sampling and a cluster random sampling.]

Controlled trials are studies in which the experimental drug or procedure 
is compared with another drug or procedure, sometimes a placebo or 
sometimes the previously accepted treatment....One way a trial can be 
controlled is to have two groups of subjects: one that receives the 
experimental procedure (the experimental group) and the other that receives 
the placebo or standard procedure (the control group).....To reduce the 
chances that subjects or investigators see what they expect to see, 
researchers can design double-blind trials in which neither subjects nor 
investigators know whether the subject is in the treatment or the control 
group.  When only the subject is unaware, the study is called a blind 
trial. (pg. 15)

[I believe that you are asking about a  historical (or, retrospective) 
cohort study.]

In medicine, the subjects in cohort studies are selected by some defining 
characteristic (or characteristics) suspected of being a precursor to or 
risk factor for a disease or health effect.(pg. 11)..[In a historical 
cohort study] the events being evaluated actually occurred before the onset 
of the study. Note that the direction of the inquiry is still forward in 
time, from a possible cause or risk factor to an outcome. (pg. 13)

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