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35 Cards in this Set

  • Front
  • Back

Epidemic spread

common source: group of persons are all exposed to an infectious agent or toxin from same source

point source: type of common source where individuals become ill within one incubation period

propagative: occurs from transmission from person to person over more than 1 incubation period

Adjusted estimation

reported cases serve as lower bound

calculations based on a model determine upper bound

midpoint is best guess to how many cases were actually seen

Types of prevention

primary prevention: prevents onset of risk behaviors --> decreases incidence (eg health education that promote healthy living)

secondary prevention: screens for risk factors and early detection of asympt or mild dz --> decreases prevalence (eg community BP screening, MD support to quit smoking)

tertiary prevention: reduces long-term impairment or disabilities --> decreases recurrences and slows dz progression (graded aerobic activity during recovery from MI)

Adjusted rates

calculated after using statistical procedures to minimize demographic differences btwn populations being compared

Prevalence rate

proportion of individuals with existing dz at a point of time

prevalence = (ppl with existing dz / population of ppl at risk for dz) x multiplier

point prevalence --> prevalence at a given time

period prevalence --> prevalence over period of time

good for chronic dz

Incidence rate

proportion of individuals developing new dz during a period of time

can only be calculated over a period of time

incidence rate = (ppl with dz onset / population at risk for dz) x multiplier

good for acute conditions

Prevalence pot

determined by rates of incidence, recovery, and death

prevalence = incidence x duration

Number needed to treat

how many ppl do you need to treat to prevent 1 case

inverse of incidence rate

if incidence 16 per 1,000 (16/1000) then NNT = 1000/16 = 62.5

if comparing two modulaties it is the inverse of absolute risk reduction (ARR) -->

1 / (incidence A - incidence B) where ARR = incidence A - incidence B

Number needed to harm

1 / (incidence of exposed - incidence of non-exposed)

Infant mortality

# deaths within 1yr of age / total births

AA have highest rates of infant mortality bc of low birth weight & infections

Native Americans have highest rate of SIDS

Infant mortality risk factors

maternal age (younger worse)


single parent family

Years of potential life lost

sum of yrs that ppl would have lived had they experienced normal life expectancy

if life expectancy is 75 & person dies at 65 then YPLL is 10

Positive predictive value

true positive / (true positive + false positive)

if test is positive what is the probability that someone has the dz

this is dependent on specificity

Negative predictive value

true negative / (true negative + false negative)

if test is negative, what is probability that someone doesnt have dz

this is dependent on sensitivity

Types of studies

Observational vs experimental studies

observational: no intervention, just observe natural course

experimental: there is an intervention and study determines the effect of the intervention

Different types of observational studies

1) case report: single clinical subject or event with no control

2) case series: characterization of a group of clinical subjects

3) cross-sectional study: presence or absence of dz determined in each member of the study population or in a representative sample at a particular time --> interested in prevalence but cause and effect cannot be determined (study is at a given time) data analyzed with chi-squard test

4) case-control study: identifies a group of ppl with a dz and group w/o dz then compares them for risk factors --> mostly retrospective (study is of the past) data analyzed with odds ratio

5) cohort study: identify population who has been exposed to risk factor & is followed over time and compared with a group not exposed to the risk factor --> prospective (study is following through the future) data analyzed with relative risk

Relative risk

goal of observational study

incidence rate of exposed group / incidence rate of unexposed group

Attributable risk

how many more cases in one group?

incidence rate of exposed group - incidence rate of unexposed group

Odds ratio

odds of exposure for cases / odds of exposure for controls

used for case-control studies

3 phases of clincial trial

Phase I: safety in healthy volunteers

Phase II: protocols and drug levels in small groups of patient volunteers (couple of 100)

Phase III: efficacy and occurrence of side effects in larger group of patient volunteers

Different types of randomized controlled clinical trials

1) double blind: neither subjects nor researchers know whether the subjects are in the tx or control group

2) community trial: therapeutic regimen received by an entire community

3) crossover study: for ethical reasons, no group involved can remain untreated; all subjects receive intervention at different times

Types of bias

selection bias: sample is not representative of population (Berkson's bias)

measurement bias: information is gathered in a manner that distorts the information (Hawthorne effect -> subject knows they are being evaluated so that changes performance)

experimenter expectancy: experimenter's expectations inadvertently communicated to subjects (Pygmalion effect)

lead time bias: gives a false estimate of survival rates

recall bias: subjects fail to accurately recall events in the past

late-look bias: individuals with severe dz are less likely to be uncovered in a survey bc they die first

confounding bias: the factor being examined is related to other factors of less interest

Variance and STD

variance = (value - mean)^2

STD = sqrt (variance1 + ... + varianceN / N)

+/-1 STD cover 68% of values are within mean

+/- 2 STD cover 96% of values are within mean

Standard error

SE = STD / sqrt (N)

Skewed curves

negatively skewed --> mean < median < mode (tail skewed to left)

positively skewed --> mean > median > mode (tail skewed to right)

Confidence interval and significance

if 95% CI overlap then not significant

if there is no overlap, then significant

Relative risk and confidence interval

if 95% CI does not contain 1.0 then is significant

if RR is >1 and CI does not contain 1.0 then there is a significant increased risk

if RR is <1 and CI does not contain 1.0 then there is a significant decreased risk

Type I (alpha) error

rejecting null hypothesis when it is really true --> think something is significant when it's not

if p-value = 0.05 --> type I error is 5 in 100 (aka 1 in 20)

Type II (beta) error

fail to reject null hypothesis when it is false --> think something isnt significant when it is

type II error= 1-power

Power of study

capacity to detect a difference if there is one

Chi-square test

you know what to expect and testing whether you are seeing that or not

eg toss coin 10x, you expect 5 heads & 5 tails


independent group: test 2 independent groups

paired sample: each group is measured 2x, once before and once after intervention

one group: have only 1 group but want to compare to imaginary group

Length time vs lead time bias

both involve screening tests

length time - testing for a dz that can have rapid or indolent progression & screening just selects for those with an indolent course thereby leading to impression screening improves survival

lead time - advent of new screening test picks up test earlier but there is no change to actual duration to mortality (increasing sensitivity picks up dz earlier)

Likelihood ratio

Positive LR = sens / (1-spec)

Negative LR = (1-sens) / spec

prob of given test result occurring in a pt with a disorder compared to the prob of the same result occurring in a pt w/o the disorder

not dependent on disease prevalence as PPV and NPV are