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# PHC 6000: Measures

### Introduction to epidemiology: Epidemiological measures

Ratio Compares two quantities
Proportion Ration where numerator is part of denominator
Rate Special proportion where time is included
T/F: Prevalence is a rate. False
Risk Proportion of unaffected individuals who eventually develop outcome. Probably of disease.
Prevalence Number of existing cases in the population
Point prevalence Number of existing cases in the population at a specific time
Incidence rate Development of new cases over time
What measure is important to use for monitoring the course of an epidemic? Incidence rate
What measure is important for determining the success of prevention/intervention efforts? Incidence rate
What measure is important for determining total disease burden and healthcare needs? Prevalence
When annual rates are reported, this may not be explicitly stated but is implied: person-years
T/F: When affected persons are no longer at risk, they cannot be included in the denominator when calculating person-time. True
What is the relationship between risk and rate? Rate can be a good estimator of risk if the event occurs once during the study, if the proportion of population affected by the events is small (>5%), or if the time interval is relatively short.
Why is rate only a good estimator of risk when the number of events affects only <5% of the population? Numbers on too large of a scale will result in a skewed risk
What is attributable risk and what information does it provide? Risk in the exposed minus the risk in the unexposed. Tells us the risk responsible for the specific exposure of interest.
Interpret the following: Rate ratio: 1.64 Exposure: Smoking Outcome: Weight control measures Smokers are 64% more likely to use weight control measures compared to non-smokers.
Interpret the following: Relative risk: 2.4 Exposure: Smoking Outcome: Heart disease Smokers are 2.4 times at risk for heart disease compared to non-smokers.
What is relative risk? Provides best estimate of the strength or magnitude of the exposure-outcome association and is useful in making causal inferences
What does RR=1 indicate? No exposure-outcome association
If risk in exposed = risk in non-exposed, what does RR equal? 1
What does RR<1 indicate? Negative association (protective)
If risk in exposed < risk in non-exposed, what does RR equal? <1
What does RR>1 indicate? Positive association (causal)
If risk in exposed > risk in non-exposed, what does RR equal? >1
What is an odds ratio? A ratio that measures the odds of exposure for cases compared to controls
What are the odds of exposure? Number exposed / Number unexposed
What is the OR numerator? Odds of exposure for cases = a/c
What is the OR denominator? Odds of exposure for controls = b/d
Interpret the following: Odds ratio: 1.62 Exposure: Smoking Outcome: CHD Those with CHD are 1.62 times more likely to be smokers than those without CHD OR Those with CHD are 62% more likely to be smokers than those without CHD
What does OR=1 indicate? There is no association between the outcome and exposure
What does OR>1 indicate? The exposure is associated with an increased risk of developing the outcome
What does OR<1 indicate? The exposure is associated with an reduced risk of developing the outcome
What is the relationship between OR and RR? If the outcome is rare, the OR (in a case-control study) is an estimate of the RR if one was to do a cohort study
Does RR vary according to the sample size of cases & controls? Yes
Does OR vary according to the sample size of cases & controls? No
What is a point estimate? A single number, calculated from a set of data, that is the best guess for the parameter
What is an interval estimate? A range of numbers around the point estimate within which the parameter is believed to fall with certain probability. Also called a confidence interval.
If the confidence interval of an OR or RR includes the value of 1.0, what can we conclude? We cannot exclude chance as being a probable explanation for the relation between the exposure association with disease
If confidence intervals of two measures/groups (e.g., comparing prevalence measures) overlap, what can we conclude? They are not significantly different
What is another name for attributable risk? Risk difference
What is risk difference? Risk exposed - risk unexposed
What is attributable risk? Risk exposed - risk unexposed
What is attributable risk percentage? (Risk exposed - risk unexposed) / Risk exposed
What are the cons of using point prevalence? Less likely to have disease on certain day because may be cleared or may not have gotten it yet
What are the pros of using period prevalence? More likely to get better picture of burden because more people likely to have had during time period
Created by: AlneciaPHS