USMLE (Fach) / Public Health Sciences (Lektion)

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• Cross-sectional study Collects data from a group of people to assess frequency of disease at a particular point in time ("snap-shot").Asks, "What is happening?" Measures disease prevalence.Can show risk factor association ...
• Case-control study Compares a group of people with disease to a group without disease.Looks to see if odds of prior exposure or risk factor differs by disease state.Asks, "What happened?" Almost always retrospective. Measures ...
• Cohort study Compares a group with a given exposure or risk factor to a group without such exposure. Looks to see if exposure or risk factor is associated with later development of disease. Can be prospective (asks, ...
• Clinical trial Experimental study involving humans. Compares therapeutic benefits of 2 or more treatments, or of treatment and placebo. Study quality improves when study is randomized, controlled, and double-blined. ...
• Sensitivity (true-positive rate) Proportion of all people with disease who test positive, or the probability that when the disease is present, the test is positive. Value approaching 100% is desirable for ruling out disease and indicates ...
• Specificity (true-negative rate) Proportion of all people without disease who test negative, or the probability that when the disease is absent, the test is negative. Value approaching 100% is desirable for ruling in disease and indicates ...
• Positive predictive value Proportion of positive test results that are true positive. Probability that a person who has a positive test result actually has the disease. PPV = TP/(TP + FP) PPV varies directly with pretest probability ...
• Negative predictive value Proportion of negative test results that are true negative. Probability that a person with a negative test result actually does not have the disease. NPV = TN/(TN+FN) NPV varies inversely with prevalence ...
• Incidence vs prevalence Incidence = # of new cases/# of people at risk (during a specified time period) Prevalence = # of existing cases/total # of people (at a point in time) Prevalence/1-prevalence = incidence rate x average ...
• Odds ratio Typically used in case-control studies. Odds ratio depicts the odds of a certain exposure given an event (eg, disease) vs the odds of exposure in the absence of that event (eg, no disease). Odds that ...
• Relative risk Typically used in cohort studies. Risk of developing disease in the exposed group divided by the risk in the unexposed group (eg, if 21% of smokers develop lung cancer vs 1% of nonsmokers, RR = 21.  ...
• Attributable risk The difference in risk between exposed and unexposed groups, or the proportion of disease occurrences that are attributable to the exposure (eg, if risk of lung cancer in smokers is 21% and the risk ...
• Relative risk reduction The proportion of risk reduction attributabe to the intervention as compared to a control (eg, if 2% of paitents who receive a flu shot develop the flu, while 8% of unvaccinated patients develop the flu, ...
• Absolute risk reduction 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 the flu vs. 2% of people who receive a ...
• Number needed to treat Number of patients who need to be treat for 1 patient to benefit.Lower number = better treatment. NNT = 1/ARR (absolute risk reduction)
• Number needed to harm Number of patients who need to be exposed to a risk factor for 1 patient to be harmed.Higher number = safer exposure. NNH = 1/AR (attributable risk)
• Selection bias Nonrandom sampling or treatment allocation of subjects such that study population is not representative of target population. Most commonly a sampling bias.  Berkson bias – study population selected ...
• Measures of central tendency Mean = (sum of values)/(total number of values)- Most affected by outliers (extreme values). Median = middle value of a list of data sorted from least to greatest- If there is an even number of values, ...
• Measures of dispersion Standard deviation = how much variability exists in a set of values, around the mean of these values.σ = SD; n = sample sizeVariance = (SD)2 Standard error = an estimate of how much variability exists ...
• Nonnormal distributions Bimodal: Suggests two different populations (eg, metabolic polymorphism such as fast vs slow acetylators; age at onset of Hodgkin lymphoma; suicide rate by age). Positive skew: Typically, mean > median ...
• Statistical hypotheses Null (H0): Hypothesis of no difference or relationship (eg, there is no association between the disease and the risk factor in the population). Alternative (H1): Hypothesis of some difference or relationship ...
• Type I error (α) Stating that there is an effect or difference when none exists (null hypothesis incorrectly rejected in favor of alternative hypothesis).Also known as false-positive error. α is the probability of making ...
• Type II error (β) Stating that there is not an effect or difference when one exists (null hypothesis is not rejected when it is in fact false).Also known as false-negative error. β is the probability of making a type ...
• Confidence interval Range of values within which the true mean of the population is expected to fall, with a specified probability. CI = mean ± Z(SE) The 95% CI (corresponding to α = 0.05) is often used. For the 95% CI, ...
• Twin concordance study Compares the frequency with which both monozygotic twins vs both dizygotic twins develop the same disease. Measures heritability and influence of environmental factors ("nature vs nurture").
• Likelihood ratio Likelihood that a given test result would be expected in a patient with the target disorder compared to the likelihood that the same result would be expected in a patient without the target disorder. ...
• Precision vs accuracy Precision (reliability): The consistency and reproducibility of a test. The absence of random variation in a test.Random error ↓ precision in a test.↑ precision → ↓ standard deviation.↑ precision ...
• Meta-analysis A method of statistical analysis that pools summary data (eg, means, RRs) from multiple studies for a more precise estimate of the size of an effect. Also estimates heterogeneity of effect sizes between ...
• Common statistical tests t-test: Checks differences between means of 2 groups.Example: comparing the mean blood pressure between men and women. ANOVA (Analysis of variance): Checks differences between means of 3 or more groups. Example: ...
• Informed consent A process (not just a document/signature) that requires:- Disclosure: discussion of pertinent information- Understanding: ability to comprehend- Capacity: ability to reason and make one's own decisions ...
• Consent for minors A minor is generally any person <18 years old. Parental consent laws in relation to healthcare vary by state. In general, parental consent should be obtained, but exceptions exist for emergency treatment ...
• Surrogate decision-maker If a patient loses decision-making capacity and has not prepared an advance directive, individuals (surrogates) who know the patient must determine what the patient would have done. Priority of surrogates: ...
• Confidentiality Confidentiality respects patient privacy and autonomy. If the patient is incapacitated or the situation is emergent, disclosing information to family and friends should be guided by professional judgment ...
• Car seats for children Children should ride in rear-facing car seats until they are 2 years old. Children should ride in car seats with a harness until they are 4 years old. Older children should use a booster seat until they ...
• Major medical insurance plants Exclusive provider organization:Providers: Restricted to limited panel (except emergencies)Specialist care: No referral required Health maintenance organization:Providers: Restricted to limited panel ...
• Healthcare payment models Bundled payment: Healthcare organization receives a set amount per service, regardless of ultimate cost, to be divided among all providers and facilities involved. Capitation: Physicians receive a set ...
• Medicare and Medicaid Medicare and Medicaid – federal social healthcare programs that originated from amendments to the Social Security Act. Medicare is available to patients ≥65 years old <65 with certain disabilities, ...
• Hospice care Medical care focused on providing comfort and palliation instead of definitive cure. Available to patients on Medicare or Medicaid and in most private insurance plans whose life expectancy is <6 months. ...
• Core ethical principles Autonomy: Obligation to respect patients as individuals (truth-telling, confidentiality), to create conditions necessary for autonomous choice (informed consent), and to honor their preference in accepting ...
• Recall bias Awareness of disorder alters recall by subjects; common in retrospective sudies. Example: Patient with disease recall exposure after learning of similar cases. Strategy to reduce bias: Decrease time from ...
• Measurement bias Information is gathered in a systematically distorted manner. Example: Association between HTN and MI not observed when using faulty automatic sphygmomanometer.- Hawthorne effect – participants change ...
• Procedure bias Subjects in different groups are not treated the same. Example: Patients in treatment group spend more time in highly specialized hospital units. Strategy to reduce bias: Blinding and use of placebo reduce ...
• Observer-expectancy bias Researcher's belief in the efficacy of a treatment changes the outcome of that treatment (aka, Pygmalion effect). Example: An observer expecting treatment group to show signs of recovery is more likely ...
• Confounding bias When a factor is related to both the exposure and outcome, but not on the causal pathway, it distorts or confuses effect of exposure on outcome.Contrast with effect modification. Example: Pulmonary disease ...
• Lead-time bias Early detection is confused with ↑ survival. Example: Early detection makes it seem like survival has increased, but the disease's natural history has not changed. Strategy to reduce bias: Measure ...
• Length-time bias Screening test detects diseases with long latency period, while those with shorter latency period become symptomatic earlier.This can lead to an apparent improvement of survival when a terminal disease ...
• Normal distribution Gaussian, also called bell-shaped. Mean = median = mode. -1σ to +1σ = 68%-2σ to +2σ = 95%-3σ to +3σ = 99.7%
• Correct result Stating that there is an effect or difference when one exists (null hypothesis rejected in favor of alternative hypothesis). Stating that there is no effect or difference when none exists (null hypothesis ...
• Pearson correlation coefficiant r is always between -1 and +1. The closer the absolute value of r is to 1, the stronger the linear correlation between the 2 variables. Positive r value → positive correlation (as one variable ↑, ...
• Decision-making capacity Physician must determine whether the patient is psychologically and legally capable of making a particular healthcare decision. Note that decisions made with capacity cannot be revoked simply if the patient ...

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