# faEpidemiology: First Aid for Step 2 CK Flashcards

Terms Definitions
 How to measure incidence. Cohort define prevalence number/proportion of cases in the population at a specific moment in time define incidence number of new cases that arise in the disease free population over a period of time if mortality of a disease decreases, what happens to prevalence? increased prevalence Population is measured for prevalence of the disease at a specific point in time. What type of study? cross-sectional study (aka prevalence study) Study which compares one diagnostic test to the gold-standard? cross-sectional study (aka prevalence study) calculate sensitivity true positives divided by total with disease (a/a+c) calculate specificity true negatives divided by total without disease (d/d+b) How to remember how to get sensitivty and specificity off a 2x2 chart? sE is before sP in alphabet, so sEnsitivity is left column and sPecificity is right column SPIn positive test in a specific test rules in the disease SnNOut negative test in a sensitive test rules out the disease calculate Positive predictive value (PPV) true positives divided by total positive results (a/a+b) calculate Negative predicitve value (NPV) true negatives divided by total negative results (d/d+c) how to organize a 2x2 table outcome/disease is always on top when do you want a highly sensitive test? when you want to screen for a disease to rule it out (usually more false positives) low disease prevalence means what for PPV and NPV? PPV goes down and NPV goes up testing someone who has risk factors means what for PPV? PPV goes up (higher pre-test odds) calculate positive likelihood ratio (+LR); remember it's a ratio of likelihoods true positive probability/false positive probability (sens/1-spec) calculate negative likelihood ratio (-LR); remember it's a ratio of likelihoods false negative probability/true negative probability (1-sens/spec) how to remember how to calculate Likelihood Ratio? (where do you put the diseased population?) truly diseased population always goes on top of the calculation: for +LR, true positives are on top, for -LR, false negatives are on top when are exposures and outcomes in a prospective cohort study? exposure in the future, outcome further in the future when are exposures and outcomes in a retrospective cohort study? exposure in the past, outcome in the more recent past define the population of study in a cohort study those who don't have the outcome but who could all potentially experience that outcome define the population of study in a case control study those who have the outcome presently Cheapest type of study. cross sectional study when are exposures and outcomes in a case control study? study group is defined by presence of outcome; exposures are measured (past or present) Cohort study: what is measured? cOhort measures Outcomes in a group with similar exposures Case control study: what is measured? casE control measures Exposures in a group with similar outcomes Case Control studies cannot measure risk. Instead they measure what? Odds Ratio: how much more likely it is for a person with outcome is to have an exposure than a person without an outcome Define Absolute Risk (AR) incidence of outcome Define Relative Risk (RR) incidence of outcome in exposed/incidence of outcome in unexposed Define Attributable Risk (aka Absolute Risk Reduction or ARR) incidence of outcome in exposed - incidence of outcome in unexposed Define Relative Risk Reduction (RRR) 1-RR Define Odds Ratio (OR) odds of exposure in person with outcome/odds of exposure in person without outcome Calculate Odds based on Probability Odds = probability of event/1-probability of event Calculate Probability based on Odds Probability = Odds/1+Odds Surgeon only operates on patients without significant comorbid conditions, then reports outcomes are better than other surgeons. What bias? selection bias Data gathered from 5 hospitals about stroke. One of the hospitals doesn't have MRI, so they use data from CT scans. What bias? measurement bias Study finds that construction workers who work 60 hours or more per week are more likely to have skin cancer. They conclude being over-worked causes skin cancer. What bias? confounding bias Patients with diarrhea are asked if they ate a specific food in the last week. Worry about what bias? recall bias New screening test claims to improve cancer survival by five years. Later study finds this cancer's average age of onset is earlier. What bias has this screening test created? lead-time bias Cancer prognosis is different in 2 hospitals. It is found that they use 2 different screeing test and one fails to catch the cancers with poor prognosis. What bias? length bias Type 1 (alpha) error vs Type 2 (beta) error: how to remember? Type 1: rejects the null Type 2: don't reject null (remember TWO is a DOUBLE negative) Define p value probability that the differences found in the study occurred by chance How to increase Power in a study. increase number of subjects How to decrease the Confidence Interval in a study? increase number of subjects Conclusion of insignificant difference based on confidence interval? (2) it crosses 1 on plot of RR or Odds it crosses 0 on plot of ARR
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