Measures in Epidemiology ● Describe the burden(measuring frequency) ○ How frequent the condition is ■ Disease ■ Risk factor ○ What is the prevalence of cigarette smoking in Canada? ● Describe the association(measure of association) ○ Is there an association between one factor with the other? ○ How strong is this association? ○ Is smoking associated with lung cancer? How strong is it associated with? ● Describe the impact (measure of impact) ○ What is the impact of exposure /intervention ○ What proportion of lung cancer is attributable to cigarette smoking? Measures of Frequency ● Prevalence ○ Measuring the existing cases in a population for a given time period ○ Expressed as proportion ■ prevalence=#cases in population/total population at risk ○ Can be… ■ point prevalence ■ Period prevalence Measures of Frequency incidence ● MEasures the number of new cases that occur in a population over a given time period ● Depending on the denominator, can be expressed as ○ Proportion(risks, cumulative incidence) ■ Measures the proportion of incident cases in the population at risk ● CI = # new cases/#at risk in population at stat of follow up ■ Any person included in the denominator must have the potential to become an incident case ■ Expressed as incidence per 1000 per year ○ Rate ( incidence density) ■ Measure of number of incident cases as a function of total amount of followup time for the population at risk ● IR=# new cases/ # person years at risk Relationship between incidence and prevalence ● Incidence and prevalence measures are related ○ duration of disease ● Prevalence= incidence Duration ● Assume constant rate in a stable population Measures of Association:Odd ratio ● Ratio>1.0 indicates odds of outcomes ○ Higher among exposed than unexposed ● Ratios= 1.0 ○ No association ● Ratios<1.0 ○ lower among exposure than unexposed ex) OR=3.2 odds mortaility is 3.2 times higher in exposed group, compared to non-exposed groups. Measures of Association:Risk ratio Risk Ratio=Cumulative incidence exposed/Cumulative incidence unexposed =(a/a+b)/(c/c+d)