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5 Background for Epidemiologic Methods
Pages 132-140

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From page 132...
... 5 Background for Epidemiologic Methods INTRODUCTION Epidemiologists use the term "risk" in two different ways to describe the associations that are noted in data. Relative Epidemiology is the study of the distribution and deter risk is the ratio of the rate of disease among groups having minants of disease prevalence in man (MacMahon and oth some risk factor, such as radiation, divided by the rate among ers 1960)
From page 133...
... Random is not the same as haphazard; a randomizing However, at relatively low doses, there is still uncertainty as device must be used, such as a table of random numbers, a to whether there is an association between radiation and dis- coin toss, or a randomizing computer program. However, ease, and if there is an association, there is uncertainty about random assignment does not guarantee comparability.
From page 134...
... . In a prospective case-control posure, and the actual dose may be estimated based on study, the cases are "incident," that is, they are selected at knowledge of items such as the X-ray equipment used the time their disease was first diagnosed.
From page 135...
... Exceptions are rare and disease. limited to situations in which some preclinical sign or symp In evaluation of the possible health effects of exposure to tom affects selection -- for example, when persons volunteer ionizing radiation, many of the informative case-control for one or another intervention because they know that they studies have been nested within cohorts.
From page 136...
... Because the risks associated with cally this is the same as asking whether d is larger than exlow levels of ionizing radiation are small, confounding bias pected, or whether b or c are smaller than expected. Accuis potentially important in low-level radiation studies.
From page 137...
... BACKGROUND FOR EPIDEMIOLOGIC METHODS 137 about whether the disease is associated with the exposure. Instead of categorizing persons with radiation exposure The rate of disease among the exposed subjects (Re)
From page 138...
... . It also plays an important role in developing the BEIR VII committee's cancer risk esti mates (Chapter 12)
From page 139...
... BACKGROUND FOR EPIDEMIOLOGIC METHODS 139 been based on the likelihood ratio, or on score statistic ap- The p-value or confidence interval that is computed estiproximations, or on computer simulations (Gilbert 1989) , mates only the likelihood that chance alone could have acwhich can lead to intervals that are not symmetric on either counted for the observed association.
From page 140...
... Analyses based on combined data provide tighter cancer. Ionizing radiation at high doses is acknowledged to confidence limits on risk estimates than analyses based on be a cause of most relatively common human cancers (IARC data from any single study population.


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