Epi Final Exam

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Risk difference

the difference btwn incidence rate of disease in the exposed group and the incidence rate of disease in the nonexposed group

Etiologic fraction

the proportion of the rate of disease in the exposed group that is due to the exposure

internal validity

proper selection of study groups and a lack of error in measurement
concerned with appropriate measurement of exposure, outcome, and association between exposure and disease

external validity

implies the ability to generalize beyond a set of observations to some universal statement

random errors

reflect fluctuations around a true value of a parameter bc of sampling variability
Contributing factors: Poor precision, sampling error, variability in measurement

Poor Precision

occurs when factor being measured is not measured sharply
analogous to aiming a rifle at an out of focus target
can be increased by increasing sample size or number of measurements

Sampling error

sample selected is not representative of the target population
increasing sample size can reduce likelihood

Systematic Errors (Bias)

Deviation of results or inferences from the truth, or processes leading to such deviation. Any trend in the collection, analysis, interpretation, publication, or review of data that can lead to conclusions that are systematically different from the truth
Contributing factors: selection bias, information bias, confounding

selection bias

relation btwn exposure and disease is different for those who participate and those who theoretically would be eligible for study but do not participate

informational bias

introduced as a result of measurement error in assessment of both exposure and disease

Recall bias

Type of information bias
better recall among cases than among controls

Interviewer/abstractor bias

type of information bias
occurs when interviewers probe more thoroughly for an exposure in a case than in a control

Prevarication (lying) bias

Type of information bias
occurs when participants have ulterior motives for answering a question and thus may underestimate or exaggerate an exposure

confounding

the distortion of the estimate of the effect of an exposure of interest because it is mixed with the effect of an extraneous factor
occurs when crude and adjusted meaures of effect are not equal
can be controlled for in data analysis

Criteria for confounders

Be a risk factor for the disease
Be associated with the exposure
Not be an intermeditae step in the causal path between exposure

Methods to Control Confounding

Randomization
Restriction
Matching

Randomization

attempts to ensure equal distribution of the confounding variable in each exposure category
Advantage: convenient, inexpensive, straightforward data analysis
Disadv.: need control over the exposure and the ability to assign subject to study groups, need large sample sizes

Restriction

may prohibit variation of the confounder in the study groups
Adv: provides complete control of known confounders
Disadv: cannot control for unknown confounders
ex. restricting participants to a narrow age category can eliminate age as a confounder

Matching

matches subjects in the study groups according to the value of the suspected or known confounding variable to ensure equal distributions
adv: fewer subjects are required, enhance validity of a follow-up study
disadv: costly b/c extensive searching and recordkeeping are required to find matches

Analysis strategies to control confounding

stratification
multivariate techniques

Stratification

analyses performed to evaluate the effect of an exposure within strata of the confounder
Adv: direct & logical, minimum assumptions, straightforward computations
Disadv: small numbers of observation, many ways to form strata with continuous variables, interpretation difficulty, categorization produces loss of info

multivariate techniques

math models that describe simultaneously the influence of exposure and other factors that may be confounding the effect
Adv: continuous variables dont have to be converted to categorical, simultaneous control of several exposure variables in a single analysis
Disadv: potential for misuse

Publication Bias

influence of study results on the chance of publication
studies with positive results are more likely to be published

screening

presumptive identification of unrecognized disease or defects by the application of tests, exampination, or other procedures that can be applied rapidly
positive screening results are followed by diagnostic tests

reliability

the ability of measuring instrument to give consistent results on repeated trials
same as precision

validity

ability of a measuring instrument to give a true measure
can be evaluated only if an accepted and independent method for confirming the test measurement exists
same as accuracy (middle of the target)
Cannot have something valid but unreliable

Sensitivity

the ability of the test to identify correctly all screened individuals who actually have the disease
(a/a+c)

specificity

the ability of the test to identify only nondiseased individuals who actually do not have the disease
(d/b+d)

predictive value (+)

the proportion of individuals screened positive by the test who actually have the disease
(a/a+b)
decreases when prevalence of disease decreases

predictive value (-)

the proportion of individuals screened negative by the test who do not have the disease
(d/c+d)
in terms of the test

accuracy of screening test

determined from 2X2 table
true positives + true negs/ all
(a+d)/(a+b+c+d)

procedures to improve sensitivity and specificity

Retrain screeners
recalibrate screening instruments
utilize a different test
utilize more that one test

lead time bias

the perception that the screen detected case has longer survival because the disease was identified early

Length Bias

particularly relevant to cancer screening
tumors identified by screening are slower growing and have a better prognosis

Selection bias

motivated participants have a different probability of disease that do those who refuse to participate

infectivity

the capacity of an agent to produce infection or disease

pathogenicity

the capacity of the agent to cause disease in the infected host
measured by the proportion of individuals with clinically apparent diseases

virulence

refers to the severity of the disease
measured by the proportion of severe of fatal cases. If fatal, use case fatality rate

Toxigenicity

the capacity of the agent to produce a toxin or poison

antigenicity

the ability of the agent to induce antibody productoin in the host. related immunogenicity.

reservoirs of infection disease

an environment that fosters the survival of infectious agents
Ex. contaminated water or food, soils, animals

portal of entry

locus of access to the human body (e.g. mouth and digestive system)
agent must exist in large enough quantities to survive in the environment and overcome the defenses at the portal of entry into the host

portal of exit

site where infectious agents leave the body (respiratory system, skin lesions)

herd immunity

immunity of a population, group, or community against an infectious disease when a large proportion of individuals are immune either through vaccinations or prior infection

attack rate

similar to incidence except only for a short period
used when the nature of the disease of condition is such that a population is observed for a short period of time
(sick)/(sick + well)

secondary attack rate

number of cases of infection that occur among contacts within the incubation period following exposure to a primary case in relation to the total number of exposed contacts
measure of contagiousness
(all cases-initial cases)/(# susceptible - initial cases)

case fatality rate

number of deaths caused by a disease among those who have the disease
# of deaths due to disease/number of cases of disease
rabies, AIDS, and Ebola have high CFR

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