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# PHC 6000: Measures

### Introduction to epidemiology: Epidemiological measures

Question | Answer |
---|---|

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