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Re: Definitions of study designs
RuthS and all,
Correction accepted with embarassment.
Cohort studies are not the same as case-control studies.
NSWS -- not same as Iowa Radon study.
In simplifying I became imprecise (but perhaps not inaccurate, because true in a broader sense).
Groups do not = individuals. I make the same argument that [type A] physicians may not be good controls for radiologists [usually typeB]
However, the lively arguments here further impel db placebo control studies, so that, "The controls should be matched as closely as possible" (Ruth S)
Howard Long
Ruth Sponsler wrote:
> Hello all -
>
> A bit of clarification is needed here. It seems that
> there is a bit of confusion regarding the definition
> of a case-control study vs. an occupational cohort
> study.
>
> At the bottom, I have provided a couple of links that
> explain some epidemiological study designs, and may
> help to clarify these designs a bit further.
>
> The most basic, "fact-finding" type of epidemiological
> study is a correlation analysis. My class project
> last semester (radon vs. life expectancy in the U.S.)
> was a simple correlation analysis. The results of a
> correlation analysis may "point to" a phenomenon that
> should be further investigated via a more tightly
> designed epidemiological study.
>
> The NSWS was not a case-control study, but rather an
> occupational cohort study. A cohort study is a
> comparative analysis of mortality of non-exposed vs.
> exposed persons. The NSWS stratified by exposure and
> by age and compared exposed vs. non-exposed shipyard
> workers. The strength of its design, as opposed to
> many other cohort studies in occupational epidemiology
> was that the study compared workers (exposed) vs.
> workers (not exposed), rather than workers vs. the
> general population. The general population contains
> more individuals disabled by serious illnesses or by
> drug/alcohol problems than do most worker populations.
> The NSWS design eliminated this 'healthy worker
> selection criterion' of being 'on-the-job' by
> comparing two groups, both on-the-job. The NSWS study
> of 8 shipyards was initiated because a small cohort
> study of one shipyard indicated an excess leukemia
> risk. The NSWS involved many more workers at 8
> shipyards and showed this result from the small study
> to be probably due to random chance.
>
> The goal of an occupational cohort study is to examine
> a cohort of workers in an industry vs.an unexposed
> cohort, matched for age and gender, to see if the
> exposed workers are experiencing an elevated risk of
> any disease from a factor or factors at work. Most
> occupational cohort studies involve analysis of death
> certificates, and some also link these certificates
> with hospital records. A non-nuclear example of a
> controlled occupational cohort study would be an
> examination of a cohort of butchers at supermarkets,
> compared vs. a cohort of supermarket personnel who did
> not work with meat i.e. checkout clerks, bakery
> employees etc. [I didn't make this up. Studies have
> actually examined this]. This design is relatively
> similar to the NSWS because workers in the same
> facility are compared with one another, the main
> difference between them being the level of exposure to
> the agent under consideration. A somewhat less
> controlled cohort study, where one would expect a
> 'healthy worker effect,' would be a comparison of oil
> refinery employees vs. the general population.
>
> A case-control study design is entirely different from
> a cohort study, and has a different goal. In a case
> control study, the researcher is trying to identify
> unique aspects of the disease case's life that may
> have put them at risk for the disease. In a
> case-control study, the researcher evaluates actual
> cases of a disease, along with controls that are
> individually matched to the cases. The controls
> should be matched as closely as possible with the
> cases with respect to factors such as age, gender,
> weight or body mass index, smoking status, alcohol
> consumption, parity (especially for studies involving
> breast cancer, not the subject here), and a number of
> other factors that may influence health status.
> Case-control studies have been discussed on the list
> extensively, as the Iowa and Missouri lung cancer
> studies are examples of this design.
>
> Perhaps a non-radiation example will clarify what a
> case-control study is. If I were to go to a
> developing country, and do a case-control study of
> diarrhea among children, then my controls would be
> kids who did not have diarrhea recently, and my cases
> would be kids with the 'runs.' I would examine the
> food, water, and living conditions of the cases vs.
> the controls. Most likely, I would find that the
> diarrhea cases had worse water supply or were perhaps
> eating food that was not prepared in a sanitary
> fashion while controls had access to cleaner water
> and/or food.
>
> The other type of study that I will briefly mention is
> the randomized controlled trial. This is an actual
> experiment. Certain types of randomized controlled
> trials are unethical, especially if the something
> being evaluated is harmful. For example, if I
> suspected that Giardia (a parasite) causes intestinal
> illness, it would be unethical for me to separate some
> nice friendly student volunteers into two groups and
> give one group a glass of water with Giardia in it and
> the other a glass of clean water. However, in certain
> instances with minor illnesses (the common cold,
> caused by rhinoviruses), randomized controlled trials,
> with consent from the subjects, have been performed to
> evaluate efficiency of transmission of the
> rhinoviruses between different settings i.e.
> temperature, how long the cold virus has 'sat' on a
> surface before the person touches his nose, etc.
>
> Randomized controlled trials are often performed in a
> double-blind fashion, where neither the patients nor
> the caregivers know which treatment is assigned to
> whom. The most common application of this study
> design is evaluation of medicines. Only the
> experiment designer knows whether the person is
> getting [silly example!] Viagra or a placebo. The
> health care providers, who do not know who is getting
> what, record the data and provide the information to
> the researchers, who then determine if there is any
> difference between the Viagra group and the placebo
> group.
>
> In all the types of studies mentioned here, statistics
> improve with improvements in matching between subject
> groups and with numbers of members in the groups. The
> key constraints are time and money.
>
> Below are links to a couple of pages that explain the
> various designs a bit further.
>
> http://cebm.jr2.ox.ac.uk/docs/studies.html
> http://bmj.com/epidem/epid.html
>
> ~Ruth 2 aka Ruth Sponsler
> >
> > Furthermore, this rejected null hypothesis and the
> > many animal and human
> > confirmations - like the 4,000 case NSWS case
> > control study - suggest
>
> > Howard Long
>
>
>
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