Epidemiology Final

Measures of Disease Frequency/Occurence

1. Prevalence

2. Risks

3. Rates

4. Odds

Prevalence

proportion of defined population that has health outcome (ex. 11% of people ages 65 and older in the US have alzheimers)

prevalence formula

number of people with health outcome/number of people in the total study population

Point Prevalence

prevalence at one period in time (ex. May 1, 2012)

Period Prevalence

prevalence over a period of time (ex. June, July, August-Summer 2012)

Prevalence Interpretation Example

Prevalence of smoking among these 500 men with lung cancer is 92.2%

Incident Cases

new cases of disease (used for risks and rates)

Risk

Measures number of new cases developing among people in population at risk over specified time period

Risk Note 1

risk is for entire study population not a single person

Risk Formula

number of incident cases/total population at risk at start of study period

Rate

measures the occurrence of new cases in a population

Rate Note

not a proportion bc denominator isn’t fixed

Rate Formula

number of incident cases/total person-time

Person-Time Stops When…

death, leaving study, person develops outcome, unable to follow-up

Person-time

Sum of time each person remains at risk for health outcome under observation

Risk Note 2

expressed as a percentage (ranges from 0% to 100%)

Odds

The ratio of the probability that an event will occur, to the probability that the event won’t occur

Odd Formula

p/(1-p)

Odds Example

If the probability of an event is 0.20, then the odds are: 0.20/(1-0.20)

Study Population

who is in your specific study

Target Population

who the results of you’re study should be applied to (greater scope)

Prevalence Count

number of existing cases in risk population during specified time frame

Incidence Count

number of new cases in risk population during specified time frame

Incidence…

assumes a closed study population

Odds Notes

describes both prevalence and incidence

unit-less

Prevalence Note

prevalence includes old + new cases

Odds Ratio

The odds of the outcome occurring in the exposed group compared to the odds of it occurring in the unexposed group

OR=1

No association

OR>1

Positive association

OR<1

negative association

Odds Ratio Formula

(a*d)/(b*c) or (a/c//b/d)

Odds Ratio Interpretation

Those who were exposed had [OR] times the odds of [outcome] compared to those who were unexposed

Odds Ratio Note 1

You can’t calculate a risk or rate from case control data as the denominator isn’t representative of the source population (investigator arbitrarily decides cases & controls)

Odds Ratio Note 2

You can use the odds ratio to estimate the risk ratio and rate ratio

RR=1

No association

RR>1

Risk in exposed is greater than in unexposed (possibly casual)

RR<1

Risk in exposed is lower than in unexposed (possibly protective)

RD=0

No association (same in both groups)

RD>0

(+) Risk in exposed is greater than in unexposed (possible casual)

RD<0

(-) Risk in exposed is lower than in unexposed (possibly protective)