Definitions of study designs

[Ruth Sponsler <jk5554@YAHOO.COM>]

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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