Behavioral Science Flashcards – First Aid for the USMLE STEP 1

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  • What type of study is a case control study?
    Observational and retrospective
  • What type of study is a Cohort study?
    Observational and prospective or retrospective
  • what type of study is a Cross-sectional study?
    Observational
  • what is the design of a case-control study?
    Compares a group of people with disease to a group without disease Looks for prior exposure or risk factor Asks what happened
  • what a measure/example of Case-control study?
    Odds Ratio (OR) patients with COPD had higher odds of hx smoking
  • what is the design of a cohort study?
    Compares a group with a given exposure or risk factor to a group without such exposure. - looks to see if exposure increases likelihood of disease - can be prospective (who will get X) or retrospective (who developed X)
  • what is a measure/example of a cohort study?
    Relative risk (RR) smokers have higher risk of developing COPD than non smokers
  • what is the design of a cross-sectional study?
    collects data from a group of people to assess frequency of disease and related risk factors at a particular point in time - asks what is happening
  • what is a measure/example of a cross sectional study?
    disease prevalence Can show risk factor association with disease but does not establish causality
  • what is the design of a twin concordance study?
    compares the frequency with which both monozygotic twins or both dizygotic twins develop same disease
  • what does twin concordance study measure?
    Heritability
  • what is the design of an adoption study?
    compares siblings raised by biological vs adoptive parents
  • what does an adoption study measure?
    measures heritability and influence of environmental factors
  • what is a clinical trial
    Experimental study involving humans - compares therapeutic benefits of >= two Tx or 1 Tx and placebo.
  • what improves the quality of a clinical trial?
    "when study is randomized, controlled, and double blinded"
  • what is the study sample in a phase I clinical trial?
    small number of healthy volunteers
  • what is the purpose of a phase I clinical trial?
    "asses safety, toxicity, and pharmacokinetics"
  • what is the study sample in a phase II clinical trial?
    small number of patients with disease of interest
  • what is the purpose of a phase II clinical trial?
    "assesses treatment efficacy, optimal dosing, and adverse effects"
  • what is the study sample in a phase III clinical trial?
    large number of patients randomly assigned either to the treatment under investigation or to the best available treatment or placebo
  • what is the purpose of a phase III clinical trial?
    compares new treatment to the current standard of care
  • what is the study sample in a phase IV clinical trial?
    postmarketing surveillance trial of patients after approval
  • what is the purpose of a phase IV clinical trial?
    detects rare or long term side effects
  • what properties of a diagnostic test are fixed?
    sensitivity and specificity
  • what properties of a test vary with prevalence or pretest probability?
    PPV NPV
  • what is the sensitivity of a test?
    "(true positive rate) portion of all people with disease who test positive, or the probability that a test detects disease when disease is present"
  • what is the formula for sensitivity (true positive rate) of a dx test?
    =TP/ (TP+FN) = 1- false negative rate
  • important things to remember about test sensitivity?
    PID= Positive in Disease SNOUT= Sensitivity rules out
  • what is a highly sensitive test good for?
    value approaching 100% is desirable for ruling out disease and indicates a low false negative rate. - used for screening in diseases with low prevalence
  • what is true of a test with 100% sensitivity?
    TP/(TP+FN)=1 FN=0 all negatives must be TN
  • what is the specificity of a dx test?
    "(true negative rate) proportion of all people without disease who test negative, or the probability that a test indicates non disease when disease is absent"
  • what is the formula for specificity of a dx test?
    =TN/(TN+FP) =1- False positive rate
  • what is important to remember about the specificity of a dx test?
    NIH= Negative in Health SPIN= Specificity rules in
  • what is true of a test that has specificity of 100%?
    TN/(TN+FP)=1 FP=0 all positives must be TP
  • what is a highly specific test good for?
    value approaching 100% is desirable for ruling in disease and indicates a low false positive rate. used as a confirmatory test after a positive screening test
  • what are the features of ELISA screening in HIV?
    sensitive high FP low threshold
  • what are the features of confirmatory western blot for HIV?
    specific high FN high threshold
  • what is PPV?
    proportion of positive test results that are true positive probability that pt actually has dx after positive test result
  • what is the formula for PPV?
    =TP/(TP+FP)
  • PPV varies with what?
    directly with prevalence or pretest probability
  • what is the implication for PPV of a high pretest probability?
    high PPV
  • what is NPV?
    proportion of negative test results that are true negative probability that pt actually is disease free if given a negative test result
  • what is the formula for NPV?
    =TN/(FN+TN)
  • NPV varies with what?
    varies inversely with prevalence or pretest probability
  • what are the implications of high pretest probability for NPV?
    high pretest probability --> low NPV
  • what is the incidence rate?
    # new cases in a specified time period / population at risk during same time period
  • important to remember about incidence?
    Incidence looks at new incidents
  • what is prevalence?
    # of existing cases / population at risk = incidence rate X average disease duration
  • important to remember about prevalence?
    prevalence looks at all current cases
  • when is prevalence > incidence?
    for chronic diseases
  • when is Odds ratio used?
    typically in case control studies
  • what is OR?
    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 was exposed (b/d) =ad/bc
  • when is Relative risk used?
    typically in cohort studies
  • what is RR?
    risk of developing disease in exposed group divided by risk in unexposed group = (a/(a+b))/(c/(c+d))
  • if prevalence is low then RR= ?
    OR
  • what is attributable risk?
    "the difference in risk between exposed and unexposed groups, or the proportion of disease occurrences that are attributable to the exposure = (a/(a+b))-(c/(c+d))"
  • what is absolute risk reduction?
    absolute reduction in risk associated with a treatment as compared to a control
  • what is the number needed to treat?
    number of patients who need to be treated for 1 patient to benefit = 1/AAR
  • what is the number needed to harm?
    number of patients who need to be exposed to a risk factor for 1 patient to be harmed = 1/attributable risk
  • what is precision?
    the consistency and reproducibility of a test the absence of random variation in a test
  • what reduces precision in a test?
    random error
  • what is the relationship between precision and standard deviation?
    increased precision --> lower standard deviation
  • what is accuracy?
    the trueness of test measurements. the absence of systematic error or bias in a test
  • what reduces accuracy in a test?
    systematic error
  • when does bias occur?
    when there is systematic error or favor in a particular direction
  • what is selection bias?
    "nonrandom assignment to participation in a study group (eg Berkson's bias, loss to follow up)"
  • what is recall bias?
    knowledge of presence of disorder alters recall by subjects; common in retrospective studies
  • what is sampling bias?
    subjects are not representative of the general population; therefore the results are not generalizable.= type of selection bias
  • what is late-look bias?
    information gathered at an inappropriate time- e.g. using a survey to study a fatal disease
  • what is procedure bias?
    "subjects in different groups are not treated the same- eg more attention is paid to treatment group, stimulating greater adherence"
  • what is confounding bias?
    "occurs when factor is related to both exposure and outcome, but is not on the causal pathway; factor confuses effect of exposure on outcome"
  • what is lead time bias?
    "early detection confused with increased survival; seen with improved screening (natural history of disease is unchanged, but early detection makes it seem as though survival is higher)"
  • what is observer expectancy effect?
    occurs when a researcher's belief in the efficacy of a treatment changes the outcome of that treatment
  • what is the Hawthorne effect?
    Occurs when the group being studied changes its behavior owing to the knowledge of being studied
  • what are the ways to reduce bias?
    blind studies to limit influence of participants and researchers on interpretation of outcomes - placebo control groups - crossover studies (each subject acts as own control) to limit confounding bias - randomization (refer to your book for more info)
  • what are the measures of central tendency?
    mean median mode
  • what are the measures of dispersion?
    Standard Deviation (SD) Standard error of the mean (SEM) Z score Confidence interval
  • what is a normal distribution?
    "Gaussian, also called bell shaped mean=median=mode"
  • what is SEM?
    s/sqrt(n)
  • what happens to SEM as sample size increases?
    SEM decreases
  • what are the features of distribution with positive skew?
    "Typically, mean > median > mode. Asymmetry with longer tail on right. Mode is least affected by outliers in the sample"
  • what are the features of distribution with negative skew?
    typically mean < median < mode asymmetry with longer tail on left
  • what is null hypothesis?
    hypothesis of no difference
  • what is alternative hypothesis?
    hypothesis of some difference
  • what is type I error (a)?
    stating that there is an effect or difference when none exists (to mistakenly accept the alternative hypothesis and reject the null hypothesis)
  • what is a?
    the probability of making a type I error.
  • what is p judged against?
    a preset a level of significance ( usually <0.05)
  • type I error is also known as what?
    false-positive error a=you sAw something that did not exist
  • what is type II error (Beta)?
    Stating that there is not an effect or difference when one exists (to fail to reject the null hypothesis when it is in fact false)
  • what is Beta?
    the probability of making a type II error
  • a type II error is AKA?
    false-negative error Beta=you were Blind to a difference that did exist
  • what is Power (1-Beta)?
    "probability of rejecting null hypothesis when it is in fact false, or the likelihood of finding a difference if one in fact exists"
  • Power increases with what?
    "increased sample size increased expected effect size increased precision of measurement ""there is power in numbers"""
  • what happens in meta-analysis?
    pools data and integrates results from several similar studies to reach an overall conclusion. high statistical power
  • meta-analyses are limited by what?
    quality of individual studies or bias in study selection
  • what is Confidence Interval?
    Range of values in which a specified probability of the means of repeated samples would be expected to fall
  • what is the formula for CI?
    CI= range from [mean-Z(SEM)] to [mean + Z(SEM)]
  • CI of 95% corresponds to what?
    p=0.05
  • "for the 95% CI, Z=?"
    1.96
  • "for the 99% CI, Z=?"
    2.58
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