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Unformatted text preview: Epidemiology Lecture 2: Population Health and Disease Measures Learning outcomes Describe measures of disease occurrence and frequency of health events frequency Calculate measures of disease related to these measures Interpret epidemiologic information Measurement & Comparison
To find out whether a community is healthy or unhealthy: health (deaths, new cases of disease, etc) community or group. first measure one or more indicators of compare the results with another Measurement & Comparison Epidemiological principles are used Selecting the best study population and comparison group Reducing or controlling for errors in measurement of the `exposure' and the `outcome' to ensure that the study is conducted in an accurate and reliable way by Types of Measures Epidemiology is concerned with presence of health problems or the occurrence of new health events in a population. counts ratios proportions rates The most common types of measures are: Count number of persons who have a particular disease Around 171,000 people in NSW have diabetes An estimated 13,120 people were living with HIV/AIDS in Australia in 2002. Types of Measures Ratio (includes proportion, percentage and rate) is obtained by dividing one quantity by another. There is no specific relationship between the numerator and the denominator. eg Number of hospital beds per 1000 population Types of Measures
Proportion a type of ratio in which the numerator is included in the denominator The ratio of a part to the whole, expressed as a ``decimal fraction'' (e.g., 0.2) as a fraction (1/5) or as a percentage (20%). eg 3.6 % NSW population have diabetes Rate An expression of the frequency with which an event occurs in a defined population. There is a relationship between the numerator and the denominator and a specified time period. Example: 0.36 per 1,000 people reported having diabetes in 199798 Measures of Disease Frequency
When measuring it is important in epidemiology to define: What is being measured? Person ie individuals included Place or location of the study population Time period of the study Measures of Disease Frequency
Example: What Person Vaccination status School children 7 10yrs Sydney Place or location live in Western Time period 2003 Measures of Disease Frequency This helps to: define the population at risk clarify relationships between the numerator and denominator for the calculation of rates and ratios identify potential associations between outcome and the factors being studied Do stroke rates differ by geographical location? Does the prevalence rate of AIDS differ by geographical location? The key measures of disease occurrence and health events: Incidence Cumulative Incidence Incidence Rate or Incidence Density Point Prevalence Period Prevalence Prevalence Incidence
Incidence = Number of new cases/events in a population, over a period of time. Example: the incidence of AIDS in the Australian population was 178 in 2001 ie 178 new cases of AIDS were diagnosed in Australia in 2001. Incidence
There are 2 main ways incidence is reported: Cumulative Incidence or attack rate Incidence rate or incidence density Cumulative Incidence Cumulative incidence (CI) =is the proportion of people in a population who became diseased or ill or experienced an event during the specified period of time. CI = No. new cases of disease or events during time period Total population at risk at the beginning of the time period Cumulative Incidence Two assumptions when calculating Cumulative Incidence: entire population at risk has been followed from the beginning of the study till the end all participants are at risk of the outcome of interest Cumulative Incidence Example 1: The daily incidence of chickenpox in first grade children at Booral primary school during the 1998 epidemic was 10 new cases per 100 children. If there were 200 children in the first grade, how many new cases would there be each day? 20 Cumulative Incidence Example 2 818 women had in vitro fertilization in NSW during 2000 80 developed a clinical pregnancy within one month of followup after the first embryo transfer procedure. The CI of pregnancy was?? CI= 80/818 9.8 cases per hundred women on the program Attack Rate Attack rate is a type of cumulative incidence applied to a narrowly defined population observed for a limited period of time, such as during an epidemic. Attack rate = No new cases of illness during a specified time period Total population at risk during that specified period The Public Health Unit was called in to Attack Rate Example Attack rate = investigate more than 20 reports of people being ill with gastroenteritis after eating at a large restaurant in Parramatta during the first week of April 2002. An investigation was conducted interviewing all patrons who ate at the restaurant during that week They found 2000 patrons ate at the restaurant that week and 400 became sick. What was the attack rate? = 400/2000 20 ill per 100 patrons Incidence Rate (Incidence Density) The incidence rate or incidence density is the number of new cases in a population divided by the total time units each individual in the population at risk was observed. Incidence Rate= No new cases of disease/events during the specified time period Sum of the length of time during which each person in the population is at risk Incidence Rate Example (Incidence Density) 4 subjects started the study in Jan 1990 In investigating the incidence of duodenal ulcer following the use of a specific drug in 14 subjects. and all finished the study in Dec 1999. Ten subjects joined the study in Dec1995 and finished the study in November 1996. During the period of observation: 5 people developed duodenal ulcer while taking the drug. Incidence Rate (Incidence Density) What is total length of time that persons were in the population is at risk (denominator)? 4 X 10 years = 40 years 10 x 1 year = 10 years Total = 50 personyears New cases = 5 What is the incidence rate of duodenal ulcers after taking the drug? Incidence rate = 5 / 50 = 10 cases per 100 person years Followup Period
Jan 1995 July 1995 Jan 1996 July 1996 Jan 1997 Duration of individual followup Subjects A
1.0 B 1.5 C X 1.0 D 2.0 E X 0.5 Zero Time Total person years at risk 6.0 Incidence Rate Can be presented in many different ways: 10 cases/1000 personyears 1 case/100 personyears 0.1 cases/10 personyears 0.01 cases/1 personyear These are all the same In many circumstances, you can assume: Incidence Rate Incidence Rate = that entry and exit from the population occurs evenly over the time period, or you only know the `average' population at risk, an approximate incidence density rate can then be estimated as: No new cases of disease/events during the specified time period (Initial population at risk + final population) /2 in the time period Mortality Rate
Mortality Rate = the incidence of deaths from all causes (all cause mortality rate) for the Australian population in one year All Cause Mortality Rate 2001 = No. new deaths during 2001 Total Aust.population at risk midyear 2001 Mortality Rate Example If there were 50,000 deaths in one year and 20,000,000 people living in Australia what will be the mortality rate for Australia in that year? Mortality rate = 50,000/20 millio 2.5 deaths per 1000 pop = 250 deaths per 100,000 pop = Mortality Rate Example
Annual Mortality Rate for Lung Cancer per 1,000 population =
No. new deaths from lung cancer per year No. of persons in the population at mid year X 1,000 Mortality Rate Example We can place restrictions on more than one characteristic simultaneously eg age and cause of death: No. new deaths from leukemia per year in children under 10 years No. of children under 10 years in the population at mid year X 1,000 rate. Mortality can be calculated over 1 year, 5 years, or longer. Note: Time must be specified in a mortality Case fatality rate (percent) Case fatality rate = No of individuals dying during a specified period of time after disease onset or diagnosis X 100 Total number of individuals with the disease during that specified period Case fatality rate Example Assume a population of 100,000 people of = 18/100,000 died of disease X in 1999 = 0.018 per 1,000 pop in 1999
What is the casefatality rate? = 18/20 who 20 are sick with disease `X', and in 1999, 18 die from the disease. What is the mortality rate? = 90% or 90 per 100 people with `X' Prevalence Prevalence is the proportion of a defined population with the disease/event of interest at a specified time period. Prevalence is a snap shot of the disease frequency at a point in time Point prevalence is the most common measure of prevalence Prevalence is often established by cross sectional surveys Prevalence
Point Prevalence = Total number of the population with the disease/event at a particular time Total population at that time Prevalence An incident case becomes a prevalent case and remains a prevalent case until recovery or death. prevalence depends on incidence and duration of disease. Where a population is in a `steady state', Thus prevalence of a disease may increase when incidence remains stable but survival of cases improves Period Prevalence Period Prevalence
= Number of the population with the disease/event at any time during a specified period Total population during that period Factors influencing prevalence rate
Increased by: longer duration of disease prolongation of life without cure increase in new cases inmigration of cases out migration of healthy people improved diagnosis Decreased by: short duration of disease high casefatality rate from disease decrease in new cases inmigration of healthy people outmigration of cases improved cure rate Prevalence Relationship between Incidence and Prevalence In a steadystate population, prevalence depends on: incidence of the condition (as all incident cases become prevalence cases) and duration of condition The relationship can be can be expressed as follows: Point prevalence = Incidence density x Duration of condition Use of Incidence and Prevalence If one wishes to look at a change in disease (eg studies of causality, acute conditions or events, outbreak investigation) use incidence. For example: Looking at the change in the incidence of cancer is important to know whether current prevention, screening and treatment activities are working. Use of Incidence and Prevalence Prevalence is used when looking at the magnitude of existing diseases usually chronic disease like diabetes where change does not occur rapidly Often both measures are used Diabetes Prevalence, US (CDC) Trends in Diabetes Prevalence, US (CDC) Comparing disease and health event rates Disease and mortality rates can be used to compare: disease/mortality rates between populations the same population over time. When comparing rates we should be The age and sex structure of two aware of the characteristics of the population(s) that may affect the rates. populations being examined may differ and these differences will affect the rates. Comparing disease and health event rates Australian population pyramid Australian indigenous population pyramid Comparison of crude death rates for indigenous Australians and all Australians in 199597 How do the population structure of the indigenous Australian population and the total Australian population compare? Comparison of crude death rates for indigenous Australians and all Australians in 199597 Are there differences between the crude deaths rates for indigenous Australian compared with all Australians? Population Crude death rates per 100,000 19951997 Males Aboriginal and Torres Strait Islander Total Australian Population 803 799 Females 559 528
(AIHW, 2001) Comparison of crude death rates for indigenous Australians and all Australians in 199597 However we know: Therefore we need to adjust for the effect of This can be done using standardization
the age structure to make a meaningful comparison. death is closely related to age the age structure of the Indigenous population is very different to that of the total population Standardization There are 2 main forms of standardisation: Direct Indirect and the calculation of standardised mortality ratios (SMRs) Direct Standardisation is used to compare large populations uses a standard reference population to compare both populations applies the agespecific disease/death rates of the population of interest to the standard population allows us to compare death rates, by calculating what their death rates would be if the populations of interest had the same age population structure as the reference population. Applying Direct Standardisation Example: Comparison of Death rates in NZ between the Maori population and the nonMaori population. Steps on Direct Standardisation 1. Select standard population = NZ Pop. 1966 (2) 2. Identify Populations of Interest: Maori population and the non Maori Population Crude death rates are: 6.37 and 9.07 3. Identify age groups breakdown (1) 4. Calculate the agespecific death rates for each of the populations you wish to compare (Maori population and the non Maori Population)=no. of deaths /population for the age group Steps on Direct Standardisation 5. Calculate the expected deaths for the Maori and nonMaori populations by multiplying the agespecific death rates of each of the Maori and nonMaori populations by the standard population (5) x (2) = (6), (7) x (2) = (8) 6. Standardised death rate the population of interest is calculated by summing the expected number of deaths for that population and divide it by the total standard population. (d) / (a); (e) / (a Age Groups Standard Population ( NZ Population 1966) Maori Population Deaths (1966)
(4) 295 Age specific rates (Per 1,000) Expected deaths Non Maori
Age specific rates /1,000 Expected deaths (1) 04 (2) 306,643 (3) 39,539 (5) = (4) /(3) 7.45 (6) = (5) x (2) 2,288 513 667 2,255 10,558 20,864 37,143 (d) (7) 3.92 (8) = (7) x (2) 1,202 221 427 1,044 5,067 15,282 23,243 (e) 514 565,756 61,728 56 0.91 0.39 1524 436,019 34,725 53 1.53 0.98 2544 640,711 43,686 153 3.52 1.63 4564 504,697 17,626 369 20.92 10.04 65+ 223,093 3,855 361 93.52 68.50 Total Crude death rate 2,676,919 (a) 201,159 (b) 1287 (c) (c) / (b) 6.37 deaths per 1000 pop Standardised Death Rate 13.9 deaths per 1000 (d)/(a) 9.07 8.7 deaths per 1000 (e)/(a) Indirect standardization and SMRs Adjusts for differences in age by calculating the number of deaths `expected' in a population, based on its age structure, if it had the same mortality experience as a reference population. Applying Indirect Standardisation Example: Comparison of Death rates in NZ between the Maori population and the non Maori population. 1. Select standard population = NZ Pop. 1966 (2) 2. Identify Populations of Interest: Maori population and the non Maori Population Crude death rates are: 6.37 and 9.07 3. Identify age groups breakdown (1) 4. Calculate the agespecific death rates for the standard population Applying Indirect Standardisation 5. Calculate expected deaths for the Maori population by multiplying the agesex specific rates for the reference population by the corresponding agesex specific populations for Maori population summing the expected deaths for Maori population 6. Calculate total expected deaths by Applying Indirect Standardisation 7. The Standardised Mortality Ratio (SMR) for the Maori population is calculated by dividing the actual number of deaths for Maori population by the expected number of deaths for Maori population x 100. population as the population of interest 8. Repeat 57 using the Non Maori Note: If the SMR is greater than 1.0, there were more deaths than expected; if the ratio is less than 1.0, there were fewer than the expected number of deaths. An SMR value of 1.67 indicates death rates 67% above that of the reference group or 1.67 times higher than expected, whereas an SMR of 97.7 indicates separation rates 2.3% below that of the reference group. Standardisation allows valid comparisons between populations with different age and sex structures Applying Indirect Standardisation Comparison of all death rates for indigenous Australians & all Australians in 199597
Population
Crude death rates per 100,000 pop Agestandardised death rates per 100,000 pop (Direct method)
Males Females Standardised mortality ratios (SMRs) (Indirect method)
Males Females Males Females Aboriginal and Torres Strait Islanders Total Australian Population (Std Population) 803 799 559 528 1840 1266 799 528 3.1 3.0 ...
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