mortality and morbidity in epidemiology

Measurement in Epidemiology: 

Measurement in Epidemiology Dr. Shruti Murthy

Introduction: 

Introduction Epidemiology- John M. Last , 1988 “ The study of the distribution & determinants of health-related states and events in specified populations and its application in the control of health problems.” 8/10/2011 2

Numerator & Denominator: 

Numerator & Denominator NUMERATOR : No. of times an event has occurred in a population in a specified time period DENOMINATOR: - related to population - related to events 8/10/2011 3

Measuring epidemiological events: 

Measuring epidemiological events Rate Ratio Proportion 8/10/2011 4

RATE: 

RATE occurrence/ change of an event in a population during a given time period No. of deaths in 1 year Mid-year population Death Rate = x 1000 8/10/2011 5

RATIO: 

RATIO A relation between 2 random quantities, obtained by dividing one quantity by the other. No. of doctors for a certain population No. of nurses for the same population Numerator NOT A PART of denominator 8/10/2011 6

PROPORTION: 

PROPORTION “A part, share, or number considered in comparative relation to a whole.” A ratio where numerator IS A PART OF of denominator No. of deaths from TB in 2010 Total deaths from all causes in 2010 x 100 8/10/2011 7

MORTALITY MEASUREMENT: 

MORTALITY MEASUREMENT Measuring death rates 8/10/2011 8

MORTALITY MEASUREMENT…: 

MORTALITY MEASUREMENT… Crude Death Rate No. of deaths during the year Mid-year population Specific Death Rates Cause – specific Group – specific Other variables- income, housing, religion… Case Fatality Ratio Total deaths due to a disease Total cases due to same disease Proportional Mortality Ratio Cause – specific Group – specific Survival Rate Proportion of survivors in a group followed over a period Adjusted or Standardized Rates Direct Standardization Indirect Standardization x 1000 x 100 8/10/2011 9

Crude Death Rate: 

Crude Death Rate No. of deaths during the year Mid – year population simplest and most straightforward summary of the population experience BUT, is crude Does not consider heterogeneity of a population Thus, cannot be used for comparing between or within population x 1000 8/10/2011 10 MORTALITY MEASUREMENT…

Specific Death Rate: 

Specific Death Rate x 1000 No. of deaths due to disease in a calendar year Mid- year population Cause -specific Group -specific x 1000 No. of deaths in a group in a calendar year Mid- year population Age, gender, age & gender-specific 8/10/2011 11

Specific Death Rate…: 

Specific Death Rate… Gives idea on etiology Permit comparison between various causes/groups within same population 8/10/2011 12 MORTALITY MEASUREMENT…

Survival rate: 

Survival rate For describing prognosis Assessing standards of treatment e.g. cancer studies x 100 Total patients alive after 5 years Total patients diagnosed/treated Calculated from the date of diagnosis 8/10/2011 13 MORTALITY MEASUREMENT…

Case Fatality Ratio: 

Case Fatality Ratio Time interval not specified Typically for acute diseases Closely related to virulence x 100 Total deaths due to a disease Total cases due to same disease Represents ‘Killing power of a disease’ Ratio of deaths to cases 8/10/2011 14 MORTALITY MEASUREMENT…

Adjusted/ Standardization of rates: 

Adjusted/ Standardization of rates Population- diverse ( sociodemographic , geographic, genetic, occupational, dietary, etc) To enable valid comparison between groups Similar to matching done in cohort studies to control for the confounding factors ‘Standardized’ -directly or indirectly depending on the data available 8/10/2011 15

Direct Standardization: 

Direct Standardization When “ standard population ” is known for which age & gender numbers are known already SDR1 = [Sum age groups (Mar Pas)]/Ps x 1000 Mar is the age-specific mortality rate for the region. Pas is the number of people in the age group in the standard population. Ps is the total standard population. 8/10/2011 16

Direct Standardization: 

Direct Standardization For each age group, you need to multiply M ag by P ag Add them up. Divide the sum by the total standard population. Multiply by 1000, or give the rate in terms of "per thousand population”. 1. M 0-5 x P 0-5 ; M 6-11 x P 6-11 ; so on till last age group ∑ (M x P) ∑ (M x P)/ T sp ∑ (M x P) x 1000 T sp 8/10/2011 17

Direct Standardization: 

Direct Standardization For each age group, you need to multiply M ag by P ag Add them up. Divide the sum by the total standard population. Multiply by 1000, or give the rate in terms of "per thousand population”. 1. M 0-5 x P 0-5 ; M 6-11 x P 6-11 ; so on till last age group ∑ (M x P) ∑ (M x P)/ T sp ∑ (M x P) x 1000 T sp 8/10/2011 18 Xcel

Indirect Standardization: 

Indirect Standardization D study [ ∑ ( M std x P study ) ] x CR std D- Deaths in study population M std - age-specific mortality rate for the standard population P study - number of people in the age group in the study population. 8/10/2011 19

Indirect Standardization: 

Indirect Standardization Age Group Number of coal workers Number of deaths (observed) Mortality rate in Reference population 40-49 3,734 37 0.005 50-59 1,887 94 0.02 60-69 1,645 327 0.1 Total 7,266 458 0.0630 8/10/2011 20

Indirect Standardization: 

Indirect Standardization Age Group Number of coal workers Number of deaths (observed) Mortality rate in Reference population Expected number deaths 40-49 3,734 37 0.005 18.7 50-59 1,887 94 0.02 37.7 60-69 1,645 327 0.1 164.5 Total 7,266 458 0.0630 220.9 8/10/2011 21