USMLE Step 3 (Subject) / Ethics and Statistics (Lesson)

First Aid Step 3

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• Nominal data Characterized by name only.No particular order to the naming, and the names are mutually exlucsive. Examples: - Blood groups; A, B, AB, O- Hepatitis types: A, B, C, D, E- HIV status: HIV-positive, HIV-negative
• Ordinal data Data set that occurs in a particular order.Although ordinal data occur in a numerical list, there are no clear breakpoints (e.g., pain scale vs. temperature). Examples:- Students' class rank- Rank list for the match
• Interval data When there are clear breakpoints, or intervals, in a set of data points. Examples:- Consider CD4: We monitor the CD4 count and stop MAI prophylaxis at 50 CD4 cells, and we stop PCP prophylaxis when it rises above 200 CD4 cells.- Speed limit- Temperature scales (freezing point, boiling point)
• Competency vs. Capacity The terms competency and capacity should not be used interchangeably.  Competency is a legal determination made only by a court, whereas capacity is a clinical assessment. Each involves the assessment of a patient's ability to think and act rationally (though not necessarily wisely). Incompetence is permanent (e.g., severe dementia) and incompetent patients are generally assigned a surrogate by the court.  Incapacity may be temporary (e.g., delirium), and careful decision making is important when considering therapeutic interventions for patients with questionable capacity.
• The principle of "double effect" Actions can have more than one consequence, some intended, others not. Unintended medical consequences are acceptable if the intended consequences are legitimate and the harm proportionately smaller than the benefit. For example, a dying patient can be given high doses of analgesics even if it may unintentionally shorten life.
• Persistent vegetative state (PVS) A clinical condition in which function of the cortex is impaired while function of the brainstem is preserved. This results in reduction of cortical function with preserved brainstem control of respiration, cardiac function, sleep-wake cycles, and reflexes.
• Sensitivity and specificity Sensitivity is the probability that a person with a disease will have a positive result on a given test.- Useful in a screening test, as the goal is to identify everyone with a given disease. Specificity is the probability that a person without a disease will have a negative result on a test.- Desirable for a confirmatory test.
• Predictive values Positive predictive value (PPV) is the probability that a person with a positive test result has the disease (true positives/all positives). If a disease has a greater prevalence, then the PPV is higher. Negative predictive value (NPV) is the probability that a person with a negative test result is disease free. A test has a higher negative value when a disease has a lower prevalence. The PPV and NPV can be determined only if the incidence in the sample is representative of the population. Generally, one needs a cohort study design to get PPV or NPV.
• Absolute risk The probability of an event in a given time period; for example, 0.1% chance of developing X in 10 years.
• Relative risk (RR) Used to evaluate the results of cohort (prospective) studies. The RR compares the incidence of a disease in a group exposed to a particular risk factor with the incidence in those not exposed to the risk factor. An RR < 1 means that the event is less likely in the expoed group; conversely, an RR > 1 signifies that the event is more likely in that group.
• Odds ratio (OR) Used in case-control (retrospective) studies. The OR compares the rate of exposure among those with and without disease. It is considered less accurate than RR, but in rare diseases the OR approximates the RR.
• Absolute risk reduction (ARR) or attributable risk (AR) Measures the risk accounted for by exposure to a given factor, taking into account the background of the disease. Useful in randomized controlled trials. Numerically, ARR = the absolute risk (rate of adverse events) in the placebo group minus the absolute risk in treated patients.
• Relative risk reduction (RRR) Also used in randomized controlled trials, this is the ratio between 2 risks. Numerically, RRR = (the event rate in control patients - the event rate in experimental patients)/the event rate in control patients. RRR can be deceptive and is clinically far less important than ARR. Consider a costly intervention that reduces the risk of an adverse event from 0.01% to 0.004%. ARR is 0.01-0.004 = 0.006%, but RRR is (0.01-0.004)/0.01 = 0.6, or 60%!
• Number needed to treat The number of patients who would need to be treated to prevent 1 event. NNT = 1/ARR.
• Statistical significance/p-value The p-value expresses the likelihood that an observed outcome was due to random chance.  A p-value < 0.05 is generally accepted as indicating that an outcome is statistically significant.
• Confidence interval Like the p-value, the CI expresses the certainty that the observation is real or is a product of random choice. Used with ORs and RR, the 95% CI says that the observed risk or odds have a 95% chance of being within the interval. Thus, the relative risk of cancer with smoking is 2.0 with a 95% CI of 1.3-3.5 – meaning that the observed RR of cancer was 2.0, and that there is a 95% certainty that the actual RR of cancer from smoking falls somewhere between 1.3 and 3.5.
• Cohort study In a cohort study, a population is observed over time, grouped on the basis of exposure to a particular factor, and watched for a specific outcome. Such studies are not good for rare conditions. Studies can be prospective or retrospective.  Use RR to interpret results. Examples include the Nurses' Health Study and the Framingham Heart Study.
• Medicare program coverage Part A:- Inpatient hospital services- Skilled nursing facility (limited)- Hospice & home health care Part B:- Outpatient physician services- Preventive care- Outpatient diagnosics (laboratory tests, x-rays)- Outpatient surgery- Hospital observation services Part C:- Medicare Advantage- Allows private health insurance companies to provide Medicare benefits Part D:- Prescription drug coverage- Provided by private insurance companies with government contracts
• HIPAA privacy rule PHIInformation pertaining to:- Patient physical or mental health- Services provided to patient- Payment for services Provider requirements- Maintain physical & device safeguards to protect PHI- Establish procedures to prevent unauthorized disclosure- Ensure compliance by workforce Breach of privacy- Impermissible use or disclosure of PHI- Provider must give written notice to patient regarding details of breach- Provider must take steps to mitigate breach & prevent future breach

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