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.
What is incidence?
NEW cases during a given period / pop at risk during that period.
Incidence is NEW incidents only.
Pts who already have it don’t count.
How can you approximate prevalence using incidence?
Prevalance is approx incidence (new cases) x disease duration
For chronic dz, which is larger, prevalence or incidence?
Prevalence is bigger than incidence
Bc duration is longer. There are more overall cases than there are new cases, bc it lasts so long
e.g. diabetes
For acute dz, which is larger, prevalence or incidence?
They are equal. Prevalence (everyone that has it) and incidence (new cases) are the same, since the dz doesn’t last long enough for prevalence to be bigger. (duration is short)
e.g. common cold
Odds ratio eqn
OR = AD / BC
What is odds ratio?
Odds of having dz in exposed divided by odds of having dz in unexposed.
Use for case-control studies (retrospective)
Relative risk eqn
RR = [a / (a+b)] / [c / (c+d)]
What is the relative risk?
Probability or getting the dz in the exposed gp vs getting the dz in the unexposed gp.
For cohort studies (prospective)
Attributable risk eqn
AR = [a / (a+b)] - [c / (c+d)]
What is attributable risk
The difference (so subtract!) in risk bt exposed [a / (a+b)] and unexposed [c / (c+d)] groups Or, the proportion of dz that is attributable to the exposure. Ex smoking causes 1/3 of the cases of pneumonia
Absolute risk reduction eqn
ARR = [a / (a+b)] - [c / (c+d)]
Note: same as AR, but now chart has risk factor Reducer rather than risk factor.
What is absolute risk reduction?
Difference in risk reduction between treatment (exposed) vs placebo (unexposed).
Same as attributable risk, but now looking at a risk factor reducer (treatment) rather than a risk factor.
What is the NNT?
1/ARR
Remember, ARR is looking at risk factor Reducers (aka TREATments) So the NNT is related to this.
Number of pts who have to get treatment before the drug successfully treats one pt. Want it to be low.
What is the NNH?
1/AR
AR looks at risk factors (HARMful things), so the NNH is related to the AR.
The number of pts who could have the risk factor before one pt was harmed. Want it to be high.
Fornicators: 30% have HPV
Non-fornicators: 5% have HPV
What is the attributable risk of fucking to getting HPV?
AR = Exposed - unexposed 30-5 = 25%
If 10,000 pts took a drug and 100 were saved by it, what is the NNT?
NNT = 100.
Need to treat 100 patients in order to save 1 patient
(10,000 drug / 100 saved = 100 drug / 1 saved)
What is precision?
Consistency and reproducibility of a test (reliability)
Absence of random variation in a test.
What is accuracy?
Trueness of test measurements (validity)
Do random error and systemic error reduce precision or accuracy?
Random error = reduced precision
Systemic error = reduced accuracy.
RR < 1
The dz is LESS likely to occur in the group
Or, the factor is protective
RR > 1
The dz is MORE likely to occur in the exposed group
Or, the factor is harmful.
RR = 1
The factor makes no difference
What is validity?
Accuracy
What is reliability?
Precision
How to reduce bias?
- blind studies (dbl blind is better)
- placebo responses
- crossover studies (switch treatment/placebo halfway through; ea subj acts as own control)
- randomization
What is bias?
One outcome is systematically favored over another. Systematic errors.
(Systematic error = reduced accuracy/validity)
Selection bias
Non-random assignment to study group
Berkson’s bias: using hospitalized pts
Recall bias
Knowing presence of disorder alters the recall by pts
Sampling bias
Subj not representative of general pop
So results can’t be generalized to general pop
Late-look bias
Info gathered at inappropriate time. This is a type of recall bias.
E.g. Giving a survey to study a fatal dz- only live pts can answer