Epidemiology and Biostats Pt. 1 Flashcards
Prevalence
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
Three types of prevention
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)
Rates vs. ratios vs. proportions
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)
How close a test measures the truth
validity
precision vs. accuracy
precision = repeatability
accuracy = overall proportion of correct tests over total predictions
if you drop a titer from 1:100 to 1:50, the test becomes more:
sensitive
what is vehicle-borne transmission?
through inanimate objects (fomites)
time between exposure and maximum infectivity
generation time
exposure time to onset of illness
incubation time
first case that brought attention to the outbreak
index case
incidence
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)
attack rate =
number of new cases/number exposed
ie. 5 people got sick out of 100 people who ate the salad
secondary attack rate =
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
crude mortality rate
proportion of individuals in a pop that die in a given period of time (usually expressed as a number per 1,000, 10,000…)
specific attack rate
number of cases in a defined subgroup
case fatality rate
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
proportionate mortality rate
deaths by specific cause / deaths from all causes
Mean
the arithmetic average.
-sensitive to outliers
Mode
the most frequently reported observation
median
the observation result that has as many observations above and below when ranked in order
morbidity rate
number with disease / number at risk
confidence interval
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
odds ratio
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
effectiveness vs. efficacy vs. efficiency
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
Triad of epi
host, agent, and environment
when are scatter diagrams best used?
used for non-linear relationships and as validity/reliability measure of continuous data
descriptive vs. analytical epi
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)
Hawthorne effect
when people in a study act differently b/c they know they are being observed
double blind vs. single blind study
single: patient doesn’t know what tx
double: patient and researcher don’t know
Berkson’s bias
a form of unrepresentative sample that gives rise to biased estimates of association
P value
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
bias
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
information bias
error due to systematic differences in the way data has been gathered from controls and cases
selection bias
error due to systematic differences between subjects selected for study and those who are excluded
recall bias
participants inaccurately recall information
observer bias
researcher records information differently for groups
How to reduce bias
-matching to decrease selection bias
-randomization
-blinding the study
Brainerd’s disease
-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
infectivity vs. pathogenicity vs. virulence
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
risk values range from:
0 to 1
attributable risk
Proportion of disease incidence or disease risk that can be attributed to a specific exposure
= incidence rate in exposed - incidence rate in unexposed
relative risk is to a ___ study as odds ratio is to a ____ study
cohort; case control
relative risk (aka risk ratio) =
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.
the problem with continuous variables
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
morbidity rate
number with disease / number at risk
point prevalence
the prevalence measured at a particular point in time; the proportion of persons with a particular disease or attribute on a particular date
period prevalence
prevalence measured over an interval of time
numerator of incidence = NEW cases that occurred during a given time period
numerator of prevalence = ALL cases present during a given time period
N/A
frequently used measures of morbidity
-incidence proportion
-secondary attack rate
-incidence rate
-point prevalence
-period prevalence
(see table page 24/79)
frequently used measures of mortality
-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
rate ratio
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
