Day 3.1 Behavioral Science Flashcards
Case-control study
Gp of people w disease vs Gp of ppl w/o disease
Retrospective (looks back in time)
Observational- looking at possible risk factors for dz (not nec a causal relationship)
Asks “What happened?”
What is the measurement for a case-control study?
Odds ration (OR = ad / bc) E.g. pts w COPD had higher odds of a hx of smoking than pts w/o COPD
Cohort study
Compares Gp w risk factor to Gp w/o risk factor to determine if risk factor increases likelihood of dz Prospective (forward in time) Observational Asks "What will happen?" Clinical trials are cohorts.
Measurement for cohort study?
Relative Risk (RR = [a/(a+b)] / [c/(c+d)] E.g. smokers have a higher risk of developing COPD than non-smokers
Cross-sectional study
Assesses freq of dx (and related risk factors) at a particular pt in time.
Observational
Asks “What is happening now?”
Measurement for cross-sectional study?
Dz prevalence
Can show risk factor association, but doesn’t show causality
Twin concordance study
Measures heritability.
Compares freq with which both monozygotic twin or both dizygotic twins develop a dz
Adoption study
Compares siblings raised by biologic vs by adoptive parents.
Measures heritability vs influence of env factors
What is meta-analysis?
Pools and integrates data from many studies to get an overall conclusion. Has greater statistical power.
“Highest echelon of clinical evidence”
Limitations: quality of individual studies; bias in study selection
2x2 table
Disease is on top, test is on side
TP FP
FN TN
pos/neg are horizontal
Sensitivity eqn
TP / [TP + FN]
Or, 1 - FN
What is sensitivity?
Proportion of all people with dz who test positive. (TP over all people w/ disease)
Also, the ability of a test to detect a dz when it is present.
SNOUT - SeNsitivity rules OUT dz
Specificity eqn
TN / [TN + FP]
Or, 1 - FP
What is specificity?
Proportion of all people w/o dz who test negative (TN over all pts w/o dz)
Also, the ability of a test to indicate non-dz when no dz is present.
SPIN = SPecificity rules IN
Are HIV tests sensitive or specific?
First test is ELISA - it’s very Sensitive but has a high FP rate (SN rules OUT, so it’s better to get everyone who might have it rather than miss someone who doesn’t).
If positive ELISA could be TP or FP, so confirm with Western Blot, which is specific, and high FN rate.
SP rules IN. So use Western as confirmatory test.
Which should be more sensitive, screening tests, or confirmatory tests? Which should be more specific?
Screening = Sensitive (SN rules OUT- so need to get all the possible positives- even if they are FP)
Confirmatory/diagnosing tests should be specific
SP rules IN.
Eqn PPV
TP / [TP + FP]
What is PPV?
The proportion of all positive results that are accurate.
Probability that pt has dz if pt has a pos test result
Eqn NPV
TN / [TN + FN]
What is NPV?
Proportion of neg test results that are correct.
Probability that a pt does not have dz, given negative result
How does high prevalence affect PPV and NPV?
If there are a lot of pts w dz, the number of TP will go up, and the number of FN will also go up (bc more pts have it, and some of those will wrongly test negative).
Increased TP means increased PPV.
Increased FN means NPV will decrease.
How does low prevalence affect PPV and NPV?
If prevalence is low, then the FP will go up, and also to TN will go up.
Increased FP means decreased PPV
Increased TN means increased NPV
Will a test be more accurate in a population with high prevalence or low prevalence?
High prevalence means a high PPV, and low NPV, so the test will peform better w high prevalence. This is why you should only screen pts who need screening.
What is (point) prevalence?
total cases / total population at risk
this is at one point in time.