# Epidemiology - Final 1

drraythe's version from 2016-02-14 01:10

## Review Pre-Quiz Basics

"Sporadic" dz occurrence is defined as?A dz that is NOT NEW & occurs infrequently (w/o regularity) in a population. Cases occur in small numbers, illness is not apparently connected w/ similar illnesses in any other animals/persons & it is not rapidly spread
Explain Epidemic vs SporadicWhilst an epidemic can be 1 case of a new dz , it is rapidly spread btwn animals /humans... BUT!!! A sporadic case refers to a person/animal whose illness is not rapidly spread & not apparently connected w/ similar illnesses in any other animals or humans
What is Incidence?A measure of the frequency w/ 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 dz over a given period of time. Incidence refers to NEW cases of dz
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 formula for Incidence?
What is the math formula for Prevalence?
What does incidence account for that prevalence doesnt?Prevalence, (unlike incidence), does not take into account the duration of dz . It is a snapshot at this point in time. (incidence keeps track of new incidences....prevalence= 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!
Less strong: Cohort & 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 w/in 1 incubation period. The graph looks like curve commonly rises rapidly & contains a definite peak at the top, followed by a decline once the point source is removed, no new cases occur
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 & may occur over >1 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
What is a "Propagating Epidemic" epidemic curve? What does the curve look like on the graph?Occurs when dz is introduced through a single (primary) source of infxn in 1 animal & then xmitted to other animals. The graph has multiple peaks & regressions
**Explain a Cohort Study. What are the 2 kinds?A cohort study is when you are looking at a group of animals/people which share a common exposure & a group which has not been exposed
There are 2 kinds of cohort studies
(1) Prospective (look at exposed & non-exposed group, follow them around & see if they develop the dz)
(2) Retrospective: Groups are still selected based on exposure (or non-exposure) & then when an individual gets the dz, medical records are used to backtrack & see if they developed the dz from the exposure... SO: The difference lies in the time in which the study begins, both start w/ exposure, but 1 looks toward the future for dz & 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 bc of some exposure, are more likely to be Dxd as a case (this is a selection bias)
What is Non-Response Bias? Is this selection or information bias?Subject’s [owners] do not participate. In 1 group all individuals respond but only a few in the other (this is a selection bias)
What is an 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- data or dz data (That results in a mistaken estimate of an exposure’s effect on the risk of dz)
What is Interviewer Bias? Is this selection or information bias?An interviewer interjects his or her bias into interview. Way questions are asked & 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 Hx (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 dz relative to un-exposed individuals? The dz is RR times more likely to occur among those exposed to the suspected risk factor than among those w/ no exposure.
**If RR is 1, less than 1, or greater than 1, what is the implication?1 = No association btwn exposure & dz
>1 = Positive association, which means it is possibly causal
<1 = Is a negative association & 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 btwn the causal/exposure factor & dz which is a major criteria for judging causal inferences
****HOW DO YOU CALCULATE A RR? WHICH STUDY DOES THIS GO W/?RR is a calculation you do w/ a COHORT study
**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 & lower boundaries of that sample RR. IF the 95% CI inclds the null value of 1.0 (no difference), then the RR could be due to chance & we say that the RR is NOT statistically significant at the 95% CI. If it doesnt incld 1, then it is significant
**What is a Case-Control Study?When you start w/ animals which already have the dz, & a group that do not have the dz. Then we compare the frequency of exposure factors in the cases w/ that of the controls. (The cases are the ones w/ the dz, the controls are the ones w/o the dz of interest).
**Do Case Controls use RR or OR?OR
****HOW DO YOU CALCULATE THE OR? WHICH STUDY DOES THIS GO W/?Odd Ratio (OR) goes w/ 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 w/ dz, c is not exposed w/ dz... b is exposed w/o dz & d is not exposed w/o dz
**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 & provide the strongest evidence of causality. There is the untreated (placebo) group & the treated group.

## Dx & Screening

What are 4 things we look for when assessing diagnostic/screening tests?The validity of the test (accuracy) (sensitivity/specificity)
The reliability of the test (reproducibility)
The predictive value of the test
The evaluation of screening programs
Explain Diagnostic vs Screening testsDiagnostic test is when you think the animal has a dz, theyre probably showing CS & the test is used to confirm/classify dz 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, positive individuals require further investigation. An example would be detection of heartworm in dogs. This test could be screening or Diagnostic, but the PTx, type & purpose of the test differ (must accommodate this in the interpretation of the results)
Validity (accuracy) of a test contains what 2 components?Sensitivity
Specificity
Define "SeNsitivity" of a testThe probability that subjects w/ dz will test positive. It is the likelihood of a positive test in a dzd animal (u can trust the Negatives)
Define "SPecificity" of a testThe probability that subjects w/o dz will test negative (u can trust the Postives)
If you have a test w/ 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
Know this chart
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?
Sensitivity equation?
What are Predictive Values?The clinical applications of sensitivity & specificity
What is the PPV (definition)Positive Predictive Value is the proportion or probability of subjects w/ a positive test results which have the dz
What is the equation for the PPV (Positive Predictive Value)?
What is the equation for the NPV (Negative Predictive Value)?
What is the NPV (definition)The proportion or probability of subjects w/ negative test results which do not have the dz?
How does Prevalence relate to Predictive Values?Predictive values are influenced by the TRUE prevalence 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?
Validity is aka?Accuracy
Reliability is aka?Reproducibility
What is screening?The examination of asymptomatic animals in order to classify them as likely/unlikely to have the dz 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 & a low level of false positive [really negative] test results
(4) Are there facilities for follow-up diagnostics + effective Tx protocols?
(5) Is the test w/o risk to the pop & is it economically justifiable?
"Natural Hx of dz" refers to? Why do we care?Course of a dz w/o intervention; from the time of exposure or biologic onset to resolution of the dz. We care bc the effects of Tx can only be ascertained if the course of the dz w/o Tx is known
What's the Preclinical phase?Time from onset to development of CS/symptoms
What’s the "Biologic onset"?No symptoms of dz (maybe change in DNA)
Do we want a long or short Preclinical phase? Why?LONG! Bc it's usually only feasible to detect + Tx a high proportion of cases early on for dz w/ a LONG preclinical phase. If it were short, virtually continuous rescreening would be necessary to meet that objective
What is the Clinical phase?The period after which signs develop
To be suitable for control by a program of early detection & Tx, a dz must pass through a...?Preclinical phase during which it is undiagnosed but detectable by screening
What is "Lead time" ?Interval by which the time of Dx is advanced (happens earlier than it normally would) by early screening (early detection of dz vs usual time of Dx)
What is a "Critical point?"A point in the natural Hx before which Tx is effective. The point after which there is no cure (These are arbitrary & different for every dz. Sometimes they happen before dx, or after, sometimes the critical point is at infxn & there is just isn't a cure or Tx)