Epidemiology Measures Notes

NOTES NOTES EPIDEMIOLOGY MEASURES DIRECT STANDARDIZATION osms.it/direct-standardization STANDARDIZATION ▪ Methods used to compare health event rates of two/more populations (e.g. mortality rates) by standardizing characteristics responsible for interpopulation differences ▪ E.g. remove confounding variables (age) when comparing two groups’ crude mortality rate (CMR) to get age-adjusted mortality rate ▫ CMR: number of people who died in one group, divided by the group population (100,000 or 1,000) DIRECT STANDARDIZATION ▪ Compares differences in health events among two/more populations by calculating age-adjusted rate ▪ Used when event distribution in each age group within population is known ▪ Process for calculating direct standardization for age-adjusted mortality rate ▫ Choose reference (standard) population (e.g. separate population such as a national-level population) ▫ Multiply other population of interest’s age-specific mortality rates to number of people in each age group of reference population ▫ Add up number of expected deaths from all age groups ▫ Calculate age-adjusted mortality rate ▫ Compare two age-adjusted mortality rates INDIRECT STANDARDIZATION osms.it/indirect-standardization ▪ Used when number of events/mortality rates in each age group within population is not known ▪ Process for calculating indirect standardization for age-adjusted mortality rate ▫ Choose reference population with known mortality rates ▫ Multiply other population of interest’s age-specific mortality rates to number of people in each age group of reference population ▫ Add up number of expected deaths from all age groups ▫ Calculate standardized mortality ratio (SMR) OSMOSIS.ORG 55

Figure 8.1 Using direct standardization to find the age-adjusted mortality rate for City 2, using City 1 as the reference population. 56 OSMOSIS.ORG

Chapter 8 Biostatistics & Epidemiology: Epidemiology Measures Figure 8.2 Using indirect standardization to find the standardized mortality ratio for City 2, using City 1 as the reference population. OSMOSIS.ORG 57

INCIDENCE & PREVALENCE osms.it/incidence-prevalence ▪ Measures number of people who have disease ▪ Reported as population percentage/ratio (e.g cases per 1000) Incidence ▪ Number of new disease cases in population over time period (usually one year) ▫ Affected by preventive measures (vaccination, diagnostic techniques) Prevalence ▪ Number of total (old, new) disease cases in population in particular time point (point prevalence) ▫ Shows disease commonness in group of people ▫ Affected by cure rate, survival rate, death rate, recurrence Relationship between incidence and prevalence ▪ New disease cases (incidence) added to amount of disease present in population (baseline prevalence) → ↑ prevalence ▪ ↑ death rate, cure rate → ↓ prevalence (↓ total disease cases) ▪ If incidence > death/cure rate → net ↑ prevalence; if incidence < death/cure rate → net ↓ prevalence ALL cases population at risk New cases iNcidence = population at risk prevALence = MEASURES OF RISK osms.it/measures-of-risk ▪ Probability that event will occur (e.g. disease development risk) Absolute risk ▪ Disease incidence in population who have been exposed to specific risk factor ▫ E.g. 1 out of 50 (2%) diabetics will develop cardiovascular disease (CVD) Absolute risk = # of events in a group # of individuals in that group Relative risk (RR) ▪ Compares disease development probability between exposed group, unexposed group ▫ E.g. smokers’ bladder cancer incidence (30%), non-smokers’ bladder cancer incidence (3%) ▫ RR = 0.3/0.03 = 10 58 OSMOSIS.ORG ▫ Smokers are 10 times more likely to develop bladder cancer Relative risk = Probability of event in exposed population Probability of event in unexposed population Absolute risk reduction (ARR) ▪ AKA risk difference ▪ Outcomes comparison (change in risk) ▫ Between population that has received treatment for a disease, population that has not received treatment ▪ ARR = risk (untreated) - risk (treated) ▫ E.g. 4% bladder cancer occurrence in group that receives particular drug, 20% in group that does not receive drug ▫ ARR = 0.2 - 0.04 = 0.16 or 16% ▫ For every 100 individuals receiving drug, 16 bad outcomes would be avoided

Chapter 8 Biostatistics & Epidemiology: Epidemiology Measures Number needed to treat ▪ Determines how many individuals should be treated with medication to prevent one person from developing bladder cancer ▫ Number needed to treat: 1/0.16 = 6.25 ▫ About six people should be treated # needed to treat = 1 Absolute risk reduction ODDS RATIO osms.it/odds-ratio ▪ Measures association between exposure (e.g. risk factor, health characteristic), outcome (e.g. disease, mortality) ▫ E.g. Which group is at higher risk of experiencing an adverse outcome? Does an intervention change risk degree for a group? ▪ Used in case-control studies: case group with identified outcome, control group without identified outcome ▪ Calculated using 2X2 frequency table ▫ Divide odds of disease in exposed individuals by odds of disease in unexposed individuals OR = OR = 40 / 20 2 = = 2.66 60 / 80 0.75 ▫ OR = 1 → exposure does not affect odds of outcome ▫ OR > 1 → exposure associated with higher odds of outcome ▫ OR < 1 → exposure associated with lower odds of outcome a / c ad = b / d bc OSMOSIS.ORG 59

ATTRIBUTABLE RISK (AR) osms.it/attributable-risk ▪ AKA risk difference/excess risk ▪ Measures difference in disease risk between exposed population, unexposed population ▫ Often used in cohort studies AR = 40 20 20 − = 0.4 − 0.2 = 100 100 100 AR × 100 incidence in exposed AR for exposed individuals AR = 20 × 100 = 50% 40 A C − A+ B C + D ▪ 50% of bladder cancer incidence → attributable to smoking in exposed population AR for population (PAR) PAR = incidence in population - incidence in unexposed PAR = 60 20 10 − = 0.3− 0.2 = 200 100 100 10 × 100 = 50% 20 MORTALITY RATES & CASE-FATALITY osms.it/mortality_rates_case-fatality ▪ Mortality rate: death incidence in population over period of time Annual mortality rate ▪ Mortality rate from all causes (crude death rate) ▫ Calculated by taking total number of 60 OSMOSIS.ORG deaths from all causes in one year divided by total number of people at risk in population at mid-year ▪ Annual mortality = total number of deaths (850) ÷ total number of people at risk in population at mid-year (500,000) = 0.0017

Chapter 8 Biostatistics & Epidemiology: Epidemiology Measures ▪ Percent: 0.0017 x 100 = 0.17% ▪ Per 100,000: 0.0017 x 100,000 = 170 (170 death per 100,000 people during year) Population-specific mortality rate ▪ Mortality rate for specific sub-population (e.g. biologically-female individuals; cancerrelated deaths, neonatal mortality) ▫ E.g. neonatal mortality rate = number of deaths among children < 28 days old (during given time interval) ÷ number of live births (during same time interval) x 1,000 Case-fatality rate ▪ Percent of people that die within certain period of time post-diagnosis ▫ Calculated by dividing number of postdiagnosis deaths by total number of diagnosed individuals, multiplied by 100 ▪ Measures disease severity Case mortality rate from disease A = 40 = 0.16 = 16% 250 DALY & QALY osms.it/DALY-QALY ▪ Disease burden measurement: impact of health problem on individual/population Disability-Adjusted Life Years (DALY) ▪ Determines disease burden according to years of life, or to compare specific intervention’s effect (e.g. new medication reducing diabetes risk) ▫ Morbidity, mortality combined into single metric ▪ DALY: years of lost life due to premature death (YLL) + years lived disability (YLD) ▫ YLL: number of deaths (N) x standard life expectancy at age of death in years (L) ▫ YLD: number of incident cases (I) x disability weight (DW) X average duration of disability in years (L) ▫ DW: reflects disease severity on 0 (perfect health) to 1 (dead) scale Quality-Adjusted Life Years (QALY) ▪ Determines disease burden according to quality of years of life, relative value of interventions (e.g. cost-utility analysis); guides healthcare-resource prioritization ▪ Measures years of life with illness/disability (considered less than year of healthy life) ▪ QALY = number of years lived x utility weight ▫ One healthy year of life = 1 QALY (1 year of life × 1 utility weight) ▫ One year of life lived in situation with illness/disability (e.g. chronic pain) = 1 year × 0.5 (utility weight) = 0.5 QALYs ▫ Death: assigned value of 0 QALYs OSMOSIS.ORG 61