# Epi- Final- 1

version from 2015-09-08 16:53

## Review pre-quiz basics

"sporatic" disease occurance is defined as...A disease that is NOT NEW & occurs infrequently (without regularity) in a population. Cases occur in small numbers, illness is not apparently connected with similar illnesses in any other animals/persons, and it is not rapidly spread
explain epidemic vs sporaticWhilst an epidemic can be 1 case of a new disease, it is rapidly spread between animals /humans... BUT!!! A sporadic case refers to a person/animal whose illness is not rapidly spread and not apparently connected with similar illnesses in any other animals or humans
((What is Incidence?a measure of the frequency with which new cases occur over a specified time period (other explanations given are:)The proportion of a population, initially free of the outcome of interest, that develops the disease over a given period of time. Incidence refers to NEW cases of disease
((What is prevalence?The number of cases that are present in a given population (other explanations given are:) Proportion of the population at a given time that have the factor of interest
((What is the math problem for incidence?I(per 1,000)= the number of new cases in a population during a particular period of time, DIVIDED BY the number of individuals at risk of developing the disease during that period of time. All of this is multiplied by 1000.
((What is the math problem for prevalence?# of individuals having a disease at a particular point in time DIVIDED BY # of individuals in the population at risk at that point in time
what does incidence account for that prevalence doesnt?prevalence, (unlike incidence), does not take into account the duration of disease. It is a snapshot at this point in time. (incidence keeps track of new incidences....prevalance= Val only takes a picture)
what is the strongest type of epidemiologic study providing EVIDENCE (NOT PROOF) that an association might be causal? What are the less strong types?A Randomized clinical trial is Best evidence for causality! Other trials which are not randomized, so are less strong are: cohort and case control, cross sectional, cases series, case report
what is a "point source" epidemic curve? what does the curve look like on the graph?animals or persons are subjected to the same BRIEF exposure over a limited, defined time period, usually within 1 incubation period. The graph looks like Curve commonly rises rapidly and contains a definite peak at the top, followed by a decline once the point source is removed, no new cases occur (see slide 14)
what is a "continuous(common) epidemic" epidemic curve? what does the curve look like on the graph?Exposure to the source is prolonged over an extended period of time and may occur over > one incubation period. On the graph, The down slope of the curve may be very sharp if the common source is removed or gradual if the outbreak is allowed to exhaust itself (see slide 16)
what is a "propagating epidemic" epidemic curve? What does the curve look like on the graph?Occurs when disease is introduced through a single (primary) source of infection in 1 animal and then transmitted to other animals. The graph has multiple peaks and regressions (see slide 20)
**explain a cohort study. What are the two kinds.A cohort study is when you are looking at a group of animals/people which share a common exposure, and a group which has not been exposed. There are 2 kinds of cohort studies: (1) prospective (look at exposed and non-exposed group, follow them around and see if they develop the dz) (2) retrospective: groups are still selected based on exposure (or non-exposure) and then medical records are used to backtrack and see if they did develop the dz from the exposure... SO: The difference lies in the time in which the study begins, both start with exposure, but one looks toward the future for dz, and and the other looks to the past for development of the dz
what is selection bias?Distortion in way subjects are selected for study
what is Surveillance bias? is this selection or information bias?persons/animals followed more closely by health care providers because of some exposure are more likely to be diagnosed as a case (this is a selection bias)
what is Non-response bias? is this selection or information bias?subjects [owners] differentially do not participate. In one group all individuals respond but only a few in the other (this is a selection bias)
what is a Inappropriate comparison group? is this selection or information bias?comparison group (controls)does not appropriately represent the population from which cases arose (this is a selection bias)
what is information bias?Systematic error in the collection of -exposure- or disease data (That results in a mistaken estimate of an exposure’s effect on the risk of disease.)
what is Interviewer bias? Is this selection or information bias?An interviewer interjects his or her bias into interview..way questions are asked and perceived by respondents affects their response (this is an information bias)
what is Recall bias? Is this selection or information bias?Ability of respondents to accurately remember exposure of pet or use of individuals other than the owner to obtain info on history (this is an information bias)
what is a case report?describes a single case
what is a case series?report which describes a group of cases
what are Cross-sectional studies? aka?aka "prevalence" studies. snapshot of health at any defined point in time or very short period of time... Description of attributes of a population based on a sampling/ SURVEY from the population.
explain how an ecological fallacy is createdAn ECOLOGICAL FALLACY is committed if an assumption is made that the association found at the herd/group level is also true on the individual level (we want to avoid this)
**What is the RR? When is it used?Relative Risk, or Risk Ratio. it is used when the following question is asked: How many more times more (or less) likely are exposed individuals to get the disease relative to un-exposed individuals?.... The disease is RR times more likely to occur among those exposed to the suspected risk factor than among those with no exposure.
**if RR is 1, less than 1, or greater than 1, what is the implication?1= no association between exposure and dz. If greater than one, it is a positive association, which means it is possibly causal. If the RR is less than one, it is a negative association, and it is possibly a protective factor against the dz
**Relative Risk ratios are used in what studies? Odds Ratios are used in what studies? what do they both measure?RR is used in cohort studies. OR is used in case-control studies. They measure the STRENGTH of the association between the causal/exposure factor and disease which is a Major criteria for judging causal inferences
****HOW DO YOU CALCULATE A RR? WHICH STUDY DOES THIS GO WITH?RR is a calculation you do with a COHORT study. You are doing this equation: [a/(a+b)] / [c/(c+d)]. LET ME EXPLAIN: The first [] is the "incidence of exposed" which is a= had exposure and dz, divided by "had exposure and dz+had exposure and not dz". This first [] is then divided by the second [], which is the incidence NOT exposed. This is c= not exposed but has dz, divided by "not exposed but has dz+ not exposed and no dz". (see slide 22 of analytical studies)
**in order to determine if the RR has statistical significance, you use the confidence interval. explain thisWhen an RR is given, the investigator should provide the 95% confidence limits for the upper and lower boundaries of that sample RR. 95% CI includes the null value of 1.0 (no difference), then the RR could be due to chance and we say that the RR is NOT statistically significant at the 95% CI. If it doesnt include 1, then it is significant.
**what is a case-control study?when you start with animals which already have the disease, or do not have the disease. Then we compare the frequency of exposure factors in the cases with that of the controls. (The cases are the ones with the dz, the controls are the ones without the dz of interest).
**do case controls use RR or OR?OR! (OR starts with o, so youd hope it was cohort bc that has an o, but epi sucks so it's the opposite)
****HOW DO YOU CALCULATE THE OR? WHICH STUDY DOES THIS GO WITH?goes with the case-control study. It is the odds that cases were exposed divided by the odds that the controls were exposed. aka [(a/c) / (b/d)], where a is exposed with dz, c is not exposed with dz... b is exposed without dz, and d is not exposed without dz (see slide 49 of analytical studies) (just remember either OR is easier than being forced to the RR (rail road) ) (This row? OR that row?)
**what is a randomized clinical trial?They test the effect of novel Therapeutic or Preventive Intervention<--INTERVENTION STUDIES! Are the choice of methods for investigation of causal hypothesis about the effectiveness of preventive measures and provide the strongest evidence of causality. There is the untreated (placebo) group and the treated group.

## Diagnosis and screening

What are 4 things we look for when assessing diagnostic/screening tests?The validity of the test (sensitivity/specificity), the reliability of the test, the predictive value of the test, and evaluation of screening programs
explain diagnostic vs screening testsDiagnostic test is when you think the animal has a dz, theyre probably showing clinical signs, and the test is used to confirm/classify disease status, guide for tx, or provide prognosis. Examples are MRI or radiography. On the other hand, screening tests are used as a "diagnostic survey" which are applied to APPARENTLY healthy/undiagnosed members of a population to detect dz. NOT intended to be diagnostic, individuals positive should require further investigation. An example would be detection of heartworm in dogs..this test could be screening OR diagnosis, but the pt. type and purpose differ (must accommodate this in the interpretation of the results)
validity(accuracy) of a test contains what two components?sensitivity, specificity
define "sensitivity" of a testthe probability that subjects with disease will test positive. It is the likelihood of a positive test in a diseased animal.
define "specificity" of a testthe probability that subjects without disease will test negative.
if you have a test with a continuous variable (such as blood pressure) that you want to compare to a gold standard, what must you do?create a "cut off" value where the test must go above the cutoff in order to be considered positive
what is a dichotomous test?a test which merely tests for positivity or negativity
look at the 2x2 table on slide 20, and understand it.yay
in a highly sensitive test, what do you trust?The negatives!! (Sensitivity, trust negative)
in a highly specific test, what do you trust?The POSITIVITES (specific, trust positive)
specificity equation?(true neg) / (true neg + false positives)
sensitivity equation?(true positive) / (true positive + false negative)
what are predictive values?the clinical applications of sensitivity and specificity
what is the PPV (definition)positive predictive value is the proportion or probability of subjects with a positive test results which have the disease
what is the equation for the PPV (positive predictive value)?(true positives) / (true positives+false positives) (so true positives divided by all positives)
what is the equation for the NPV (negative predictive value)?(true negatives) / (true negatives+false negatives)
what is the NPV (definition)the proportion or probability of subjects with negative test results which do not have the disease
how does prevalence relate to predictive values?predictive values are influenced by by the TRUE prevalance of a dz in a population. The higher the prevalence, the higher the positive predictive value (statistically, this just makes sense)
what is the equation for prevalence?(total disease or Ab positive) / (total population)
validity is aka?accuracy
reliability is aka?reproduciblity
what is screening?the examination of asymptomatic animals in order to classify them as likely/unlikely to have the disease that is the object of screening
Screening must be feasible. What are the 5 major criteria for a screening program to be feasible?(1) Is the dz an important public health problem? (2) Is there an established gold standard test w/ good sensitivity, specificity, PV that can detect before CS are evident? (3) The screening program should lead to a high level of case detection and a low level of false positive [really negative] test results (4) Are there facilities for follow-up diagnostics + effective treatment protocols? (5) Is the test w/o risk to the pop and is it economically justifiable?
To be suitable for control by a program of early detection and treatment, a disease must pass through a...?preclinical phase during which it is undiagnosed but detectable by screening
"Natural History of Disease" refers to? Why do we care?course of a disease without intervention from>time of exposure or biologic onset to resolution of the disease. We care because the effects of treatment can only be ascertained if the course of the disease without treatment is known
What's the Preclinical phase?Time from onset to development of clinical signs/symptoms
What is "lead time" ?Interval by which the time of diagnosis is advanced by early screening (early detection of disease vs usual time of diagnosis)
what is a "critical point?"A point (s) in the natural history before which treatment is effective. If a disease is curable, cure is possible before this point, not after.
what is the clinical phase?the period after which signs develop
whats the "Biologic onset" ?no symptoms of dz (maybe change in DNA)
do we want a long or short preclinical phase? Why?LONG! because it's Usually only feasible to detect + treat a high proportion of cases early on for dz with a LONG preclinical phase. If it were short, virtually continuous rescreening would be necessary to meet that objective