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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