Behavioral Science from USMLE First Aid
Terms
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Woman with anxiety about a gynecologic exam is told to relax and to imagine going through the steps of the exam.
What process does this exemplify?  Systematic desensitization.

Large group of people is followed over 10 years. Every 2 years, it is determined who develops heart disease and who does not.
What kind of study is this?  Cohort study.

Girl can groom herself, hop on one foot, and has an imaginary friend.
How old is she?  Four years old.
 Observational study that choses samples based on presence or absence of disease; collects information about risk factors.
 Case control study.
 Observational study where sample is chosen based on presence or absence of risk factors. Subjects followed over time for development of disease.
 Cohort study.
 Pooling data from several published studies to achieve greater statistical power.
 Metaanalysis.
 Experimental study comparing therapeutic benefits of 2 or more treatments, or treatment versus placebo.
 Clinical trial.
 Selection bias
 Nonrandom assignment to a study group.
 Recall bias
 Knowledge of presence of disorder alters recall by subjects.
 Sampling bias
 Subjects are not representative of the group; therefore results are not generalizable.
 Latelook bias
 Information is gathered at an inappropriate time.
 Prevalence of disease
 P = total cases at one time / total population
 Incidence of a disease
 I = new cases over a given time / total population AT RISK during that time
 Sensitivity

Number of true positives divided by number of all people with the disease.
Probability of a positive test given that a person has the disease.  False negative rate
 Equal to 1  sensitivity.
 False positive rate
 Equal to 1  specificity.
 Specificity

Number of true negatives divided bynumber of all people without a disease.
Probability of a negative test given that a person is free of the disease.  Positive predictive value (PPV)

Number of true positives divided by the number of people who tested positive for the disease.
The probability of having a condition given a positive test.  Negative predictive value (NPV)

Number of true negatives divided by number of people who tested negative for the disease.
The probability of not having the condition given a negative test.  Odds ratio (OR)

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OR = ad/bc
Odds of having disease in exposed group divided by odds of having disease in unexposed group.
Approximates relative risk if prevalence of disease is not too high.
Used for casecontrol studies.  Relative risk

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RR = [a/(a+b)]/[c/(c+d)]
Disease risk in exposed group divided by disease risk in unexposed group.
Risk is calculated within a group as number with disease divided by total number of people in group.
Used for cohort studies.  Precision
 Precision is the consistency and reproducibility of a test (reliability); absence of random variation
 Accuracy
 Accuracy is the trueness of test measurements (validity)
 Null (Ho)

Hypothesis of no difference
(There is no association between the disease and the risk factor in the population.)  Alternative (H1)

Hypothesis that htere is some difference
(There is some association between the disease and the risk factor in the population  Type I error (alpha)

Stating that there is an effect or difference when none exists
P is judged against alpha, a preset level of significance (usually <.05); where P is the probability of making a type I error