Basic Measurement in Epidemiology PDF

1 Basic Measurement in Epidemiology By: Firdawek G.  2 Learning objective At the end of this session, the students will be able: Describe tools used to quantifying health related events Calculate & interpret the measures of morbidity Calcul...

1 Basic Measurement in Epidemiology By: Firdawek G.  2 Learning objective At the end of this session, the students will be able: Describe tools used to quantifying health related events Calculate & interpret the measures of morbidity Calculate & interpret the measures of mortality  3 Contents  Count, ratio, proportion, rates  Measures of Morbidity/disease  Measures of mortality/death  4  Epidemiology is mainly a quantitative discipline, so we should quantify/measure the occurrence health related events Why we quantify/measure occurrence health related events ? 1. To be familiar with its distribution and make comparisons (to identify population at a higher risk) 2. For monitoring the health status of the population and planning health services(appropriate intervention)  5 Tools used in quantifying disease/death frequency are: 1.Count 2.Ratio 3.Proportion 4.Rate  6 1.Simple Count  It counts health related events and expresses as integers  It Answers:  how many people have this health related events or number of cases in a defined population  Can be used to compare disease/death frequency between populations iff the population are known to be of similar size E.g.≠of malaria cases in town A  7 2.Ratio A ratio is the relative size of two quantities or one character divided by another  Is expressed as : a/b or a:b or ‘a’ to ‘b’  No specific relationship is necessary between the numerator(a) and denominator(b)  Ranges from zero to infinity  8 Example Assume that in region X there are1,383 nurses who are serving for 2,076,408 population. What is nurse to population ratio? =total population /total nurses =2,075,408/1,383=1,501  9 3. Proportion  It measures occurrence of a part of event to the whole population  It is a ratio in w/c numerator is included in the denominator  Is expressed as :a/b  be expressed as a fraction,decimal,or percentage  Its result ranges between 0 and 1 or (0–100%)  10 Example Imagine, 130,176 of142,515 households in malarias areas, were sprayed with indoor residual spray,what proportion of households in malarias areas were sprayed? =Total HH sprayed with Indoor Residual Spray Total households in malarias areas =130,176/ 142,515=91.3%  11 4.Rate  Special form of proportions that includes a specification of time  Measures speed at w/c things happen  Rate may be expressed in any power of 10  Values range from zero to 1  12 Notice three important aspects of this formula: 1.The persons in the denominator must reflect the population from which the cases in the numerator arose 2.The counts in the numerator and denominator should cover the same time period. 3.In theory, the persons in the denominator must be “at risk” for the event  13 Example Assume that of 70,026,113 population at risk for malaria, 3,303,469 new cases of malaria were found in country A in 2022.What is the rate of malaria per 1,000 populations at risk = ≠of new malaria cases in country A in 2022 *1000 Total population at risk for malaria =≠3,303,469 * 1000 70,026,11 =47.2  14 NB All rates are ratios All rates are proportion too  All proportions are ratios  But all proportions are not rates All ratios are not proportions  15 Measures of disease occurrence Disease occurrence 1.Prevalence A.Point Prevalence B. Period prevalence 2. Incidence A.Cumulative incidence/ incidence proportion B.Incidence density/rate  16 1. Prevalence (P)  Is the measure of the proportion of persons having a particular disease in a specific population at a given point in time or over period of time  Is expressed as :P=A/B Where A= total number of cases=new case + pre-existing case B= total number of individuals/population  A measure of disease status/burden/  17 Types of Prevalence 1.Point prevalence Measures number of cases that exist in a population at a given single point in time Data is collected at one point in time Point prevalence = Number of cases in the population at time t x 10n total population at time t  18 Example Health professionals made interview to all 500 students in school X on September 10/2024 and found 50 of them had symptoms of influenza. What is the period prevalence of influenza? =50 cases of influenza/500 students =10%  19 2.Period prevalence Measures number of cases that exist in a population during a specified period of time ( week, month, year,…) provide a more accurate picture of the overall prevalence , because: Data is collected over a period of time(2 dates) It combines both old and new cases as well as it considers those who were cured or died during the period  20 Period prevalence =all cases (pre-existing + new) during the time period x10n average population during the given period of time  21 Example A study was made among 5,000 people. Of these, 125 had the disease of interest at the beginning. Over the next one year, 75 developed the disease. Calculate the prevalence of disease 1.at the beginning of the year 2.at the end of the year  22 Solution 1.at the beginning of the year Point prevalence=125/5,000*100=2.5% 1.at the end of the year Period prevalence=125+75/5,000*100=4%  23 Example 2 The figure in the next slide represents 10 cases of illness over about 15 months in a population of 20 persons. Each horizontal line represents one person. The down arrow indicates the date of onset of illness. The solid line represents the duration of illness. The up arrow and the cross represent the date of recovery and date of death, respectively  25 Calculate 1.the prevalence on October 1, 2004 2.the prevalence on April 1, 2005. 3.the prevalence from October 1, 2004, to September 30, 2005.  26 Calculate the prevalence on October 1, 2004 Given  6 persons were ill (1,2, 4, 5,8, and 9)  20 population on that date =6/20*100=30%  27 Calculate the prevalence on April 1, 2005 Given  7 persons were ill (1, 4, 5,6, 7, 9, and 10)  18 population on that date (2 died before April 1, 2005) =7/18*100=38.9%  28 Calculate the prevalence from October 1, 2004, to September 30, 2005 Given  10 persons were ill 20 population on the dates =10/20*100=50%  29 Consider when you compare prevalence: 1.Point prevalence cannot be compared with period prevalence 2.Prevalence may be compared among different disease or population if the same length of time is used  30 2. Incidence  Measures the occurrence of new events/cases in a defined population at risk within a specified period of time  population at risk is persons who are potentially susceptible the event being studied  Can be measured as proportion/rate  Measures probability/risk/rapidity of developing disease/events during a period of observation  31 A. Cumulative Incidence/proportion  It measures occurrence of new cases of disease or events in a population at risk over a specified period of time  It assumes that the entire population is at risk and is followed up for specified time of period  Population at risk is fixed and followed for the same time  It is an estimate of average risk  32 CI =≠of new events during a specified period of time x10n Population at risk in the specified period of time  33 Example Two surveys were done in the same community 12 months apart. Of 5,000 people surveyed the first time, 25 had antibodies to histoplasmosis. Twelve months later, 35 had antibodies, including the original 25. Calculate 1. the prevalence at the second survey, and 2. Compare the prevalence with the 1-year incidence  34 1. Prevalence at the second survey: x = antibody positive at second survey = 35 y = population = 5,000 x/y X10n = 35/5,000 x 1,000 = 7 per 1,000 2. Incidence during the 12-month period: x = number of new positives during the 12-month period = 35 - 25 = 10 y = population at risk = 5,000 - 25 = 4,975 x/y x10n = 10/4,975 x1,000 = 2 per 1,000  35 Activity In 2011 E.C, Arba Minch University enrolled 2000 students in the freshman program. During entry to the university in Sept 2011, voluntary testing for HIV was made and 10 were positive for HIV. A year later,re-test was made on the remaining students and 20 become positive. In Sept 2013, during re-testing, another 20 converted positive. (Assume no one has died, or has been dismissed or taken withdrawal)  36 1. What is the prevalence of HIV/AIDS on Sept 2013? 2. What is the prevalence of HIV/AIDS between Sept 2011 and Sept 2013? 3. What is the cumulative incidence of HIV/AIDS between Sept 2011 and Sept 2012?  37 Special type of incidence Attack Rate An attack rate is a variant of cumulative incidence When to apply/use? in a narrowly defined population in a narrowly defined time ,such as during an outbreak  38 Attack rate =#new cases among the population during the period ×100 Population at risk at the beginning of the period Example Of 100 persons who attended a weeding ceremony, 46 subsequently developed gastroenteritis. Calculate the AR AR =46/100*100=0.46=46%  39 Secondary Attack Rate A secondary attack rate is a measure of the frequency of new cases of a disease among the contacts of known cases Secondary Attack Rate = #cases among contacts of primary cases × 100 total number of contacts  40 Example Seven cases of hepatitis A occurred among 70 children attending a child care center. Each infected child came from a different family. The total number of persons in the 7 affected families was 32. One incubation period later, 5 family members of the 7 infected children also developed hepatitis A. Calculate 1. the attack rate in the child care center 2. the secondary attack rate among family contacts  41 Answer 1.Attack rate cases = 7 number of children = 70 Attack rate =7/70*100=0.1=10% 2. Secondary attack rate cases among family contacts = 5 total contacts = 32 − 7 = 25 Secondary attack rate =5/25*100=0.2=20%  42 Activity Consider an outbreak of shigellosis in which 18 persons in 18 different households all became ill. The population of the community was 1,000.One incubation period later, 17 persons in the same households as these “primary” cases developed shigellosis. If the 18 households included 86 persons, calculate: A.Attack rate B.Secondary attack rate  43 B. Incidence density /rate  It measures occurrence of new events over a specified period of time in a population at risk throughout the interval IR =Number of new events during a period Total person-time of exposure/sum of time periods at risk  Denominator: the sum of each individual’s time at risk (or time free of a disease) is counted  44 Person- time  It is a calculation combining persons and time  It is sum of length of time period passed free of illness (at risk) by each individual member of study  It accounts for the amount of exposure time of members  45  Individuals may be exposed to the risk of an event for varying amounts of time during a total time period of a certain length due to:  Entering the time period later  Leaving the time period earlier  Experiencing the event of interest  It is a good measure in a dynamic cohort  46 Example 1000 HIV negative persons were tested one year later and 50 were found HIV positive. 1.What is the cumulative incidence of HIV infection? 50 cases per 1000 population are at risk or(5% are at risk) in this year  47 2.What is the incidence density of HIV infection? Assuming disease is acquired on the midpoint of the interval between the last disease-free visit and the first visit when disease is diagnosed 950 persons not infected = 950 person-years 50 persons at risk for ½ year = 50 x ½ = 25 person-years =50 new cases/975 person-years = 0.05 case per person-year, or 5.1 cases per 100 person-year  48 Example Investigators enrolled 2,100 men in a study and followed them over 4 years to determine the rate of heart disease. We assume that persons diagnosed with disease and those lost to follow-up were disease-free for half of the year, and thus contribute ½ year to the denominator  49 1.Initial enrolment: 2,100 men free of disease 2.After 1 year: 2,000 disease-free, 0 with disease, 100 lost to follow-up 3.After 2 years: 1,900 disease-free, 1 with disease, 99 lost to follow-up 4.After 3 years: 1,100 disease-free, 7 with disease, 793 lost to follow-up 5.After 4 years: 700 disease-free, 8 with disease, 392 lost to follow-up  50 1. Identify the cases diagnosed 2. Calculate the person-years of observation 3.Calculate person time rate solution 1. cases diagnosed = 1 + 7 + 8 = 16 2. person-years of observation  51 Or 1)700 men x 4.0 years = 2,800 person-years 2)8 + 392 = 400 menx3.5 years = 1,400 person- years 3)7 + 793 = 800 menx2.5 years = 2,000 person- years 4)1 + 99 = 100 menx1.5 years = 150 person-years 5)0 + 100 = 100 menx0.5 years = 50 person-years Total = 6,400 person-years of observation 52  53 Comparing Incidence & Prevalence Incidence Prevalence  It is all new+ pre-  It is new cases over existing cases at period of time point/period of time  Indicate how fast the  Indicate how much is disease happening the disease there  Measures disease risk  Measures disease  Useful for identifying burden risk factors  Useful for planning health services  54 Relationship of Incidence to Prevalence  Prevalence is based on both incidence (risk) and duration of disease.  High prevalence of a disease within a population may reflect high risk, or it may reflect prolonged survival without cure.  Conversely, low prevalence may indicate low incidence, a rapidly fatal process, or rapid recovery  Thus, prevalence rate is directly proportional to both incidence rate and to the average duration of the disease and thus expressed as p ~ IR x D Relationship between prevalence and incidence  56 Factors influencing Prevalence Increased By  By longer duration of the disease  Prolongation of life of patients without cure  Increase in new cases  In-migration of cases  Out-migration of healthy people  In-migration of susceptible people  Improved diagnostic facilities(better reporting)  57 Decreased By  Shorter duration of the disease  High case fatality  Decrease in new cases  In-migration of health people  Out-migration of cases  Out-migration of susceptible people  Improved cure rate of cases  58 Measures of Mortality  Mortality is the fundamental factor in the dynamics of population growth  A mortality rate is a measure of the frequency of occurrence of death in a defined population during a specified interval Mortality rate = deaths occurring during a given time period × 10n size of the population among which the deaths occurred  59  Variety of measures of death : 1. Crude death rate 2. Specific death rate  60 1.Crude death rate Rates that apply on total population/summary rate It is the mortality rate from all causes of death for a population. Is influenced by individual population characteristics Used for International comparisons (superficial)as being populations are not similar in all respects Easy to calculate  Difficult to interpret b/c of being crude  61 2.Specific death rates Rates calculated with in homogeneous subgroups (age,sex,…) Mortality rate limited to a particular group Example ≠of deaths occurred among persons age 10-19yrs in 2020 total number of persons age 10-19yrs in 2020 Used for detail understanding of death among population subsets/risk factors 62 63  64 Examples In 2016 E.C,city A has 200,000 inhabitants. Four hundred of them had disease X.There were 1,000 deaths in one year. Of1000 deaths, 25 died from disease X. Calculate: 1.crude mortality rate 2.cause-specific mortality rate 3.death-to-case ratio 4.Proportionate mortality  65 Crude death rate =Total number of deaths during a given time interval*10n Mid-interval population =1000/200,000*1000 =5 per 1,000 population  66 Cause-specific mortality rate =≠ of deaths assigned to a specific cause *10n Mid-interval population =25/200,000*100,000 =12.5 per 100,000 population  67 Death-to-case ratio =≠ of deaths assigned to a specific cause *10n Number of new cases of same disease =25/400*100 =6.3 %  68 Proportionate mortality =≠ of deaths assigned to a specific cause *10n Total number of deaths from all causes =25/1000*100 =2.5 %  69 Thank you for your attention! Any concern, question?