|MadSci Network: Medicine|
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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