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? 
crosssectional study (aka prevalence study)

Study which compares one diagnostic test to the goldstandard? 
crosssectional 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 pretest odds)

calculate positive likelihood ratio (+LR); remember it's a ratio of likelihoods 
true positive probability/false positive probability (sens/1spec)

calculate negative likelihood ratio (LR); remember it's a ratio of likelihoods 
false negative probability/true negative probability (1sens/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) 
1RR

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/1probability 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 overworked 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? 
leadtime 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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