Measures of Morbity_Mortality & Association PDF

MEASURES OF DISEASE OCCURRENCE MORBIDITY MEASURES  Frequency: Disease Occurrence N u m e r a t o r ( events, cases ) Proportion Ratio Rate Event x...

MEASURES OF DISEASE OCCURRENCE MORBIDITY MEASURES  Frequency: Disease Occurrence N u m e r a t o r ( events, cases ) Proportion Ratio Rate Event x Time D e n o m i n a t o r ( sample, population )  Ratio  A ratio is a relative size of two quantities, calculated by dividing one quantity into another  The comparison of any two values Thenumerator and denominator need not be related.  E.g. Male: Female sex ratio; Apple and Oranges 3  Proportion  Comparison of a part to a whole  Numerator MUST BE INCLUDED in the denominator A proportion may be expressed as a decimal, a fraction, or a percentage.  Often expressed as % A ------------  Percentage = proportion x 100 A+B 4....e.g. of rate.... # of deaths in 2022 Death rate = _________________ x 1000 mid year population Mid-year population Used because pop size changes daily due to: Birth Death Migration Mid-year pop estimate on 1st July of a year. 5 5  Risk Definition: proportion of an initially disease free population that develops disease during a specified (usually limited) period of time # new cases during a specified period size of population at start of period Synonyms: - "Attack rate" 6  MORBIDITY MEASURES RATIO: expresses relationship between two numbers e.g. x:y or x/y X k. example male: female PROPORTION: This is a specific type of ratio in which the numerator is included in the denominator and the resultant value is expressed as a percentage  MORBIDITY MEASURES RATE is a special form of proportion that includes specification of time. CRUDE RATE: Summary rates based on the actual number of events in total population over a given time period. SPECIFIC RATES : Summary rates based on the actual number of events in a population subgroup over a given time period.  INCIDENCE INCIDENCE measure the probability that health people will develop a disease during a specific period of time; i.e. it is the number of new cases of a disease in a population over a period time IR =No. of new cases of disease x time p. Population at risk  Incidence Occurrence of new cases Number of new cases Population at risk overtime A measure of absolute risk Involves three components -New cases -Population at risk -Interval in time Can be expressed as a proportion or rate  Incidence Proportion Incidence per 1000= Number of new cases during specified time period *1000 Number of persons at risk  AT RISK Individuals are at risk of disease if they: - do not have the disease at the start of the follow up period -Are capable of developing the disease -have the organ of interest -have not been immunized against the disease  Example of Cumulative Incidence 200 people free of disease get observed for 3 years 10 cases of disease develop 3-year cumulative incidence=10/200=0.05  Incidence Rate (Incidence Density) Incidence per 1000= No. of NEW cases of a disease occurring in the population during specified time period*1000 Total person-time of observation  Incidence Rate (Incidence Density) Average rate at which a disease develops in a population over a specified time period Is a true rate and has the units of time Ranges from 0 to infinity Accounts for differing lengths of follow-up Assumes risk of developing disease is constant over time  Person Time Sum, over all individuals, of time at risk until the event of interest, death, loss to follow-up, or the end of the study  Example of Incidence Density Subject Follow-up( yrs) Died 1 2 N 2 2 Y 3 1 N 4 1 N 5 1 Y 6 3 N 7 1 Y 8 1 Y 9 1 N 10 2 Y 15 5  Incidence Density-Example Incidence Density=5 deaths 15 person-years =0.33 deaths per person-year =33 deaths per 100 person-years  Incidence density-Example Among 14 exclusively breastfed infants, there were 18 episodes of pneumonia over a total follow-up period of 287 child months. Incidence rate =(18/287)x100 =6.27 episodes of pneumonia per 100 child months of follow-up  CALCULATING INCIDENCE - REQUIREMENTS Need to follow prospectively a defined group of people and determine the rate at which new cases appear  CALCULATING INCIDENCE - REQUIREMENTS a) Knowledge of the health status of the study population: b) Determine time of onset c) Specification of numerator i.e. number of persons versus number of conditions. If numerator is not specified then it is taken as persons (risk per person)  CALCULATING INCIDENCE - REQUIREMENTS d) Specification of denominator – always a defined population - Use population at midpoint for time - Only those at risk form the denominator e) Always state definite period of time. (period of observation)  PREVALENCE Measures the number of people in a population who have the disease at a given time PR =No. existing cases of disease x time Total Population Prevalence depends on a) The number of people who have been ill in the past (previous incidence) b) The duration of their illness  PREVALENCE POINT PREVALENCE – measures the probability of people having a disease at a given point in time PERIOD PREVALENCE = Prevalence at a point in time, plus new cases (incidence) and recurrences during a succeeding period (e.g. one year)  REQUIREMENTS FOR CALCULATING PREVALENCE Similar to those for incidence EXCEPT a) Knowledge of time of onset is not required b) Denominator always includes the entire related population since numerator contains both old and new cases  Measures at Population Level incidence prevalence All (old + new) cases mortality emigrations + recoveries (in a dynamic/population model)  USES OF PREVALENCE Determine workload, especially in chronic diseases Planning of facilities and manpower needs To express the burden of some attribute or condition in a population To monitor control programmes for chronic conditions; e.g. mental illness (reflects duration as well as incidence)  USES OF PREVALENCE To estimate importance of a disease in a population (calculation of incidence) Useful in tracking changes in disease patterns over time (can determine point prevalence by a series of cross sectional surveys  USES OF INCIDENCE Fundamental tool for aetiologic studies of both acute and chronic diseases (as direct indicator of risk of disease) it is the basis for statements about probability or risk of disease NB: Incidence is direct measure of risk. High prevalence does not necessarily signify high risk. (it may signify increase in survival) Low prevalence may reflect a rapidly fatal process or rapid cure of disease as well as low incidence.  Measures of Mortality Mortality refers to death (cf morbidity for illness)  MORTALITY RATES Annual (crude) mortality rate =Total number of deaths from all causes in one year/number of persons in the population at midyear x 1,000 Age-specific mortality rate =Number of deaths from all causes in children under five years old in one year/number of children less than 5 years old in the population at midyear x 1,000  MORTALITY RATES Disease or cause-specific mortality rate =Number of deaths from cancer in one year/number of persons in the population at midyear x 1,000 Case-Fatality rate (percent) =Number of individuals dying during a specified period of time after disease onset or diagnosis/individuals with the specified disease x 100  MORTALITY RATES Case-Fatality rate (percent) =Number of individuals dying during a specified period of time after disease onset or diagnosis/individuals with the specified disease x 100  Example: Case Fatality Rate Assume a population of with 1000 deaths in the year. In one year, 20 individuals became sick with cholera and 6 died from the disease The cause-specific cholera mortality rate for that year was: 6/1000 = 0.006 = 0.6% The case-fatality rate from cholera for that year was: 6/20 = 0.3 = 30%  MORTALITY is an index of the severity of a problem both from the clinical and public health standpoint may also be used as an index of risk of disease e.g. when case fatality is high and when the duration of illness (survival) is short. Measures of Association  Epidemiologic Measures Measures of disease frequency - measures disease risk or burden in a population – Prevalence – Incidence  Measurement of Risk Information about the risk of contracting a disease is of great value. The knowledge that something is a risk factor for a disease can be used to help: Prevent the disease. Predict its future incidence and prevalence. Diagnose it (diagnostic suspicions are higher if it is known that a patient was exposed to the risk factor). Establish the cause of a disease of unknown etiology.  Epidemiologic Measures Measures of association – Calculations used to measure disease frequency relative to other factors – Indications of how more or less likely one is to develop disease as compared to another Epidemiologic Measures of Association Absolute – Risk difference exposed - unexposed Relative – Risk ratios – Odds ratios exposed / unexposed  Epidemiologic Measures of Association The relative risk of myocardial infarction in men compared with women is : 5 Riskmen 5 cases/1000 PY Risk ratio = = = 5 Riskwomen 1 case/1000 PY The absolute risk difference between men and women is : 4 cases/1000 PY 5 cases/1000 PY - 1 case/1000 PY = 4 cases/1000 PY  Epidemiologic Association Statistical relationship between two or more events, characteristics, or other variables Statistical relationship between exposure and disease Association is not causation!  Risk Factor A factor (exposure) found to be associated with a health condition an attribute or exposure that increases the probability of occurrence of disease – behaviour -- time – genetic -- person – environmental -- place – social Epidemiologic Measures of Association Relative risk Odds ratio Attributable risk/population attributable risk percent Standardized mortality ratios  2 x 2 Tables in Epidemiology Used to summarize frequencies of disease and exposure and used for calculation of association Disease Yes No Total Exposure Yes a b a+b No c d c+d Total a+c b+c a+b+c+d  2 x 2 Tables: Contents of Cells a = number of individuals who are exposed and have the disease b = number who are exposed and do not have the disease c = number who are not exposed and have the disease d = number who are both non-exposed and non-diseased  2 x 2 Tables in Epidemiology Used to summarize frequencies of disease and exposure and used for calculation of association Disease Yes No Total Exposure Yes (exposed) a b total # exposed No (unexposed) c d total # unexposed Total total # total # Total Population with with no disease disease  Relative Risk The ratio of the risk of disease in persons exposed compared to the risk in those unexposed Often, a measure of association between incidence of disease and exposure of interest Incidence rate of disease in exposed RR = Incidence rate of disease in unexposed  Disease Exposure Yes No Total Yes a b a+b No c d c+d Total a+c b+c a+b+c+d a / (a + b) Relative Risk = c / (c + d)  Relative Risk Develop Do Not Totals Incidence CHD Develop per CHD 1000/yr Smokers 84 2916 3000 28.0 Non- 87 4913 5000 17.4 smokers Incidence in smokers = 84/3000 = 28.0 Incidence in non-smokers = 87/5000 = 17.4 Relative risk = 28.0/17.4 = 1.61  Interpretation of Relative Risk 1 = No association between exposure and disease – incidence rates are identical between groups > 1 = Positive association – exposed group has higher incidence than non- exposed group < 1 = Negative association or protective effect – non-exposed group has higher incidence – example:.5 = half as likely to experience disease  A relative risk of 1.0 or greater indicates an increased risk A relative risk less than 1.0 indicates a decreased risk  Odds Ratio The ratio of the odds of a condition in the exposed compared with the odds of the condition in the unexposed Usually applied to prevalence studies rather than incidence studies odds of disease in exposed OR = odds of disease in unexposed  Disease Yes No Total Exposure Yes a b a+b No c d c+d Total a+c b+c a+b+c+d [a / (a + b)] / [1 – (a/(a+b))] Odds Ratio = [c / (c + d)] / [1 – (c/(c+d))]  Odds Ratio Disease Yes No Total Exposure Yes a b a+b No c d c+d Total a+c b+c a+b+c+d [a/b] [ ad ] Odds Ratio = [c/d] = [ bc ] Based on the Odds Ratio formula, what is the Odds Ratio for each disease status in this famous smoking study? Smoking and Carcinoma of the Lung Disease # of # of smokers nonsmokers Status Males 647 2 Lung cancer Males 622 27 Controls Females 41 19 Lung cancer Females 28 32 Controls Doll R. Bradford, Hill A. Smoking and carcinoma of the lung: preliminary report. British Medical Journal 1950, 2: 739-748.