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How is epidemiology defined by the text?

As the "study of the distribution and determinants of states of health and illness in human populations."


What statistical means are used to describe various health and illness phenomena?

Ratios, proportions, and rates


What does a ratios express?

The relationship between two numbers by dividing the numerator by the denominator; ratio= a/b


What does a proportion represent?

A fraction in which the numerator is a subset of the denominator; proportion = a/ (a+b); converted to percentages


What is a rate?

A proportion expressed over a particular unit of time; used to express the change in a health variable in the population at risk over a certain period


How do epidemiologist use ratios, percentages, and rates?

To express prevalence and incidence


What does prevalence express?

Proportion of a population that exhibits a certain condition at a given point in time; prevalence= existing cases/population examined at a given point in time


What is incidence?

The rate of new cases of a condition that develop during a specific period of time; incidence = new cases during time period / population at risk during time period


What is the relationship between incidence and prevalence?

The relationship depends on the nature of the condition being examined. For disease and injury that are of short duration, incidence is often greater than prevalence. For conditions that are of long duration, incidene is often lower than prevalence.


What are the three forms of rates used to compare the sample with the population norms?

Crude rates- rates calculated using the entire population at risk; specific rates- rates for specified subgroups of the population; adjusted rates- used when one wishes to compare rates across populations with different proportions of various subgroups


What is relative risk?

the probability that different populations with different characteristics ill be affected by disease or injury in some way


What are the two ways in which relative risk is calculated?

using risk ratios and odds ratios


How is a risk ratio calculated?

by creating a ratio of the incidence rate for one subgroup and the incidence rate for another subgroup; risk ratio= [a/(a+b)] / [c/(c+d)]


How is an odds ratio calculated?

used to estimate relative risk among subgroups; odds ratio= (a/c)/(b/d)= ad/bc


What are the four key proportions used to compare usefulness of the test being evaluated to a criterion test that is considered the "gold standard"?

sensitivity, specificity, positive predictive value, and negative predictive value


What is sensitivity?

the proportion or percentage of individuals with a particular diagnosis who are correctly identified as positive by the test; sensitivity= a/(a+c)


What is specificity?

the proportion or percentage of individuals without a particular diagnosis who are correctly identified as negative by the test (rate of correct rejections); specificity= d/(b+d)


How are Receiver-operative characteristics (ROC) curves used?

incorporate both the physical ability and the response criterion of a psyophysical decision into one curve


What is used to calculate a likelihood ratio?

sensitivity and specificity; likelihood ratio of a positive test= sensitivity%/ (100-specificity%); likelihood ratio of a negative test= (100-sensitivity%)/ specificity %


What is the positive predictive value?

percentage of individuals identified by the test as positive who actually have the diagnosis; positive predictive value= a/(a+b)


What is the negative predictive value?

the percentage of those identified by the test as negative who actually do not have the diagnosis; negative predictive value= c/(c+d)


What are three common nonexperimental epidemiological designs?

cross-sectional, case-control, and cohort designs


How are cross-sectional designs used?

to document the health status of a group at a particular point in time


How are case-control designs used?

researchers identify individuals with the condition of interest and individuals without the condition of interest then look to past and present for risk factors; start with "effects" and looks for "causes"


How are cohort designs used?

researchers identify individuals with various risk factors and look into the future to see if the condition of interest develops


What are some advantages to the case-control study?

that ability to study a reasonable number of individuals with relatively rare conditions without needing to observe the entire population at risk for long periods, enables researchers to study things that might not otherwise be ethical to study


What is one of the main concerns with case-control studies?

the manner in which the presumed causes are documented or identified; not all factors may be well documented