Epidemiology and Biostats Pt. 1

Question 1

Prevalence

Answer 1

number of animals in a population at one point in time that have a particular disease (a proportion)

high prevalence means that risk is high and/or disease is chronic

low prevalence means risk is low and/or animals die or recover quickly

Question 2

Three types of prevention

Answer 2

1) primary (preventing exposure to causal factors; quarantine and vaccination)
2) secondary (screening tests to detect dz before it occurs)
3) tertiary (treatments once you have the dz)

Question 3

Rates vs. ratios vs. proportions

Answer 3

Rates = a/a+b over TIME
(the frequency with which an event occurs in a defined population in a period of time)

Ratios = a/b where a is not part of b
(the relative magnitude of 2 quantities or a comparison of any 2 values)

Proportions = a/a+b but no time element
(number of events with a particular characteristic/total number of events in which the numerator is a subset)

Question 4

How close a test measures the truth

Answer 4

validity

Question 5

precision vs. accuracy

Answer 5

precision = repeatability
accuracy = overall proportion of correct tests over total predictions

Question 6

if you drop a titer from 1:100 to 1:50, the test becomes more:

Answer 6

sensitive

Question 7

what is vehicle-borne transmission?

Answer 7

through inanimate objects (fomites)

Question 8

time between exposure and maximum infectivity

Answer 8

generation time

Question 9

exposure time to onset of illness

Answer 9

incubation time

Question 10

first case that brought attention to the outbreak

Answer 10

index case

Question 11

incidence

Answer 11

number of new cases in a population specific time
-usually expressed as a proportion or rate with a denominator: the probability of occurrence of a given condition in a population within a specified period of time (ie. 5 cases/10,000/year)

Question 12

attack rate =

Answer 12

number of new cases/number exposed
ie. 5 people got sick out of 100 people who ate the salad

Question 13

secondary attack rate =

Answer 13

of new cases occurring within the incubation period following ID of index case in a household, family or close contact / the susceptible # of persons exposed to the index case during the same time interval

Question 14

crude mortality rate

Answer 14

proportion of individuals in a pop that die in a given period of time (usually expressed as a number per 1,000, 10,000…)

Question 15

specific attack rate

Answer 15

number of cases in a defined subgroup

Question 16

case fatality rate

Answer 16

cumulative incidence of death in the group of individuals that develop dz over a time period. Number of deaths of those that have the disease.

ie. 30 puppies died of parvo of 100 that were diagnosed = 30% case fatality rate

Question 17

proportionate mortality rate

Answer 17

deaths by specific cause / deaths from all causes

Question 18

Mean

Answer 18

the arithmetic average.
-sensitive to outliers

Question 19

Mode

Answer 19

the most frequently reported observation

Question 20

median

Answer 20

the observation result that has as many observations above and below when ranked in order

Question 21

morbidity rate

Answer 21

number with disease / number at risk

Question 22

confidence interval

Answer 22

range of values that has a probability of containing the parameter being estimated
-width of CI indicates amount of variability inherent in the estimate and thus the effect of sample size
-increases as data becomes more variable
-gets tighter as sample size increases
-95% and 99% most commonly used

Question 23

odds ratio

Answer 23

a measure of association between an exposure and an outcome in a case-control study. Represents the odds an outcome will occur given a particular exposure, compared to the odds of an outcome occurring in the absence of that exposure.

Calculation: OR = ad/bc

a = exposed + disease
b = exposed + no disease
c = not exposed + disease
d = not exposed + no disease

Question 24

effectiveness vs. efficacy vs. efficiency

Answer 24

effectiveness: the ability of the program to produce the intended result in the field
efficacy: ability to produce results under IDEAL conditions
efficiency: ability to produce intended results with a minimum expenditure of time and resources

Question 25

Triad of epi

Answer 25

host, agent, and environment

Question 26

when are scatter diagrams best used?

Answer 26

used for non-linear relationships and as validity/reliability measure of continuous data

Question 27

descriptive vs. analytical epi

Answer 27

descriptive: provides info on the pattern of disease (who, what)
analytical: concerned with identifying or measuring the effects of risk factors, or is concerned with the health effects of specific exposures (how, why)

Question 28

Hawthorne effect

Answer 28

when people in a study act differently b/c they know they are being observed

Question 29

double blind vs. single blind study

Answer 29

single: patient doesn’t know what tx
double: patient and researcher don’t know

Question 30

Berkson’s bias

Answer 30

a form of unrepresentative sample that gives rise to biased estimates of association

Question 31

P value

Answer 31

the probability that the effect observed could of resulted from chance alone
ie. <0.05 means there is no more than a 5% chance the observation is due to chance

Question 32

bias

Answer 32

the result of a systematic error in the design or conduct of the study that results in a mistaken estimate of an exposure’s effects on the risks of disease
-2 main types: information and selection bias

Question 33

information bias

Answer 33

error due to systematic differences in the way data has been gathered from controls and cases

Question 34

selection bias

Answer 34

error due to systematic differences between subjects selected for study and those who are excluded

Question 35

recall bias

Answer 35

participants inaccurately recall information

Question 36

observer bias

Answer 36

researcher records information differently for groups

Question 37

How to reduce bias

Answer 37

-matching to decrease selection bias
-randomization
-blinding the study

Question 38

Brainerd’s disease

Answer 38

-syndrome of acute onset of severe frequent watery diarrhea
-can occur as outbreaks
-can last for a month or longer
-cause is unknown! even after extensive research

Question 39

infectivity vs. pathogenicity vs. virulence

Answer 39

infectivity: the proportion of exposed people who become infected

pathogenicity: the proportion of infected people who become clinically ill

virulence: proportion of infected people who become clinically ill as determined by immunoassay

Question 40

risk values range from:

Answer 40

0 to 1

Question 41

attributable risk

Answer 41

Proportion of disease incidence or disease risk that can be attributed to a specific exposure

= incidence rate in exposed - incidence rate in unexposed

Question 42

relative risk is to a ___ study as odds ratio is to a ____ study

Answer 42

cohort; case control

Question 43

relative risk (aka risk ratio) =

Answer 43

incidence rate of exposed / incidence rate of unexposed

= (a/a+b)/(c/c+d)

if relative risk = 1, risk in both groups is equal and you accept null hypothesis that there is no difference. If relative risk > 1, risk of exposed is greater than underexposed. If relative risk <1, risk in exposed is less and it has a protective effect.

Question 44

the problem with continuous variables

Answer 44

a cut-off must be established so those who fall below are negative and those who fall above are positive. Cut-off must be chosen to sacrifice either sensitivity or specificity

Question 45

morbidity rate

Answer 45

number with disease / number at risk

Question 46

point prevalence

Answer 46

the prevalence measured at a particular point in time; the proportion of persons with a particular disease or attribute on a particular date

Question 47

period prevalence

Answer 47

prevalence measured over an interval of time

Question 48

numerator of incidence = NEW cases that occurred during a given time period

numerator of prevalence = ALL cases present during a given time period

Answer 48

N/A

Question 49

frequently used measures of morbidity

Answer 49

-incidence proportion
-secondary attack rate
-incidence rate
-point prevalence
-period prevalence

(see table page 24/79)

Question 50

frequently used measures of mortality

Answer 50

-crude death rate
-cause-specific death rate
-proportionate mortality
-death to case ratio (number of deaths assigned to a specific cause during a given time interval)
-neonatal mortality rate
-postneonatal mortality rate
-infant mortality rate
-maternal mortality rate

Question 51

rate ratio

Answer 51

compares the incidence rates, person-time rates, or mortality rates of 2 groups. The 2 groups are typically differentiated by demographic factors or by exposure

= rate for group of primary interest/rate for comparison group

rate ratio > 1 when there is increased risk for the group in the numerator

rate ratio < 1 when there is decreased risk for the group in the numerator

when there are equal rates in the 2 groups, the rate ratio will be 1