Epidemiology/Biostats Flashcards
Purpose of case-control study
Compares a group of people with disease to a group without disease–What happened?
Gives Odds Ratio
Purpose of Cross-sectional study
Assesses frequency of disease ata a particular point in time. Asks: What is happening?
Cohort Study
Compare a group with a given exposure/risk factor to a group without such exposure
What is the purpose of a cohort study?
It compares a group with a given exposure of risk factor to a group without such exposure to see if the exposure INCREASES the likelihood of diseas–Who will develop the disease?
Relative risk (RR)
Twin concordance study
Compares frequency with which both monozygotic twin or both dizygotic twins develop same disease
Measures heritabilty and influence of environmental factor
Define sensitivity (true-positiv rate)
Its the proportion of all people with a disease who test positive, or, its the probability that a test detects disease when the disease is present.
What does it mean when sensitivity (true-positive rate) is 100%?
A value approaching 100% is desirable for RULING OUT disease and indicates a LOW FALSE-NEGATIVE RATE.
High sensitivity test used for screening in diseases with low prevalence.
What is the equation for sensitivity (true-positive rate)?
True-positive rate = (TP)/(TP+FN)
TPr = 1 - FNr
What is the definition for specificity (true-negative rate)?
Its the proportion of all people without disease who test negative, or the probability that a test indicates non-disease when the disease is absent.
What does it mean when specificity (true-negative rate) is 100%?
A value approaching 100% is desirable for RULING IN a disease and indicates a LOW FALSE-POSITIVE RATE.
High specificity test used for confirmation after a positive screening test.
If specificity is 100%, all positives must be TPs.
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What is the equation for specificity (true-negative rate)?
TNr = (TP)/(TN+FP) TNr = 1 - (false-positive rate)
What is the positive predictive value (PPV)?
It’s the proportion of positive test results that are true positive, or, the probability that a person ACTUALLY has the disease given a positive test result
What’s the equation for positive predictive value (PPV)?
PPV = (TP)/(TP+FP)
PPV is proportional to _________.
Prevalence (or) pretest probability
What is the definition of a negative predictive value (NPV)?
It is the proportion of negative test results that are true negative, or, the probability that a person ACTUALLY is disease FREE given a negative test result.
What is the equation for negative predictive value (NPV)?
NPV = (TP)/(TP+FP)
NPV varies _________ with prevalence or pretest probability.
inversely
Equation for incidence rate
Incidence rate = (# of new cases in a specified time period)/(population at rate during same time period)
Equation for prevalence
Prevalence = (# of existing cases)/(population at risk)
Prevalence looks at all current cases
Prevalence __(,=)__ incidence for chronic diseases
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Odds ratio (OR) are typically used in ________ studies.
case-controlled
Define Odds ratio (OR); give its equation
It is the odds that the group with the disease (cases) was exposed to a risk factor (a/c) divided by the odds that the group without the disease (control) was exposed (b/d)
What is the equation for Odds Ratio?
OR = (a/c)/(b/d) = ad/bc
Define relative risk (RR)
It’s the risk of developing disease in the exposed group divided by risk in the unexposed group
(eg - if 21% of smokers devlop lunch cancer vs 1% of nonsmokers, RR = 21/1 = 21)
What’s the equation for relative risk (RR)?
RR = (a/[a+b])/(c/[c+d})
Define relative risk reduction and give its equation
Its the proportion of risk reduction attrivutable to the intervention as compared to a control.
RRR = 1 - RR
(eg - if 2% of patients who receive a flu shot develop flu, while 8% of unvaccinated patients develop the flu, then RR = 2/8 = 0.25, and RRR = 1 - RR = 0.75)
Define attributable risk
Its the difference in risk between expoed and unexposed groups, or the proportion of disease occurrences that are attributable to the exposure
(g - if risk of lung cancer in smokers is 21% and risk in nonsmokers is 1%, then 20% (or .20) of the 21% risk of lunch cancer in smokers is attributable to smoking)
What is the equation for Attributable risk (AR)?
AR = [ (a)/(a+b) ] - [ (c)/(c+d) ]
What is the definition for absolute risk reduction (ARR)?
It’s the difference in risk (not the proportion) attributable to the intervention as compared to a control.
(eg - if 8% of people who receive a placebo vaccine develop flue vs 2% of people who receive a flue vaccine, the ARR = 8% - 2% = 6% or 0.06)
What is the ‘number needed to treat’?
Number of patients who need to be treated for 1 patient to benefit. Calculated as 1/ARR
What is the ‘number needed to harm’?
It’s the number of patients who need to be exposed to a risk factor for 1 patient to be harmed. Calculated as 1/AR
