Measures of Health & Disease PDF

http://www.bized.co.uk Measures of Health & Disease: Frequency, association & impact Edward T. Dassah 1 Outline Measures of disease frequency Measures of association/effect of exposures Relevant public health measures (measures of impact) Sources of public health data 2 Objectives Define & calculate a range of measures of frequency of disease Define, calculate and understand the application of measures of association between risk factors and disease Define and calculate measures of impact Select appropriate measures for different contexts Appreciate the various sources of public health data 3 Measures of disease frequency (prevalence, risks, rates, odds) 4 Introduction Epidemiology involves: estimating the frequency and distribution of diseases in populations and comparing the effect of suspected risk factors on the frequency of diseases 5 Measures of disease frequency Used to describe how common an illness is in a population in a time frame. Key elements include; – cases (event/outcome of interest) – size of a population (the population at risk) – time (period during which the events are observed) 6 Case: Definition A case is the person in a population who has a particular disease, or to whom the event of interest/outcome occurs. In short, a case could be; infection disease (morbidity) disability death (mortality) recovery presence of antibodies and even utilization of health services 7 Prevalence & Incidence Two main types of measures of disease frequency: – Incidence : occurrence of new cases – Prevalence: frequency of existing cases – New cases are called incident cases – Existing cases are called prevalent cases. 8 Prevalence Prevalence: Point prevalence is the proportion of people in a defined population (N) who have the outcome of interest (d). Measures existing, or prevalent cases of a disease only measure of disease occurrence that can be obtained from cross-sectional study Example: The prevalence of HIV in adults in Africa in 2005 was 7.2% 9 Incidence Frequency of new cases of disease in a defined population during a specified time period Three measures of incidence: risk, odds & rate – For rare disease, risks, odds and rates are numerically similar Example: In the last academic year, 5% of students of Queen’s Hall had at least 1 episode of malaria 10 Prevalence and incidence https://www.youtube.com/watch?v=1jzZe3ORdd8 https://www.publichealth.hscni.net/node/5277 11 Table: 2x2 table classifying study subjects according to their exposure and disease status 12 Risk or cumulative incidence Risk is the proportion of individuals in a population (N), initially disease free, who develop the disease within a specified time interval (d). It is therefore the probability of getting the disease in a given time period. Using Table 1: 13 What proportion of the population at risk are affected after 5 months? 14 After 1 month, incidence=2/200=1/100 O O 15 At 2 months, incidence=1/200, prevalence=3/200 O O O 16 In the 5th month, incidence=2/200, prevalence=6/200 O O O O O O 17 Odds The odds of disease to non-disease, is the ratio of people who get the disease (d) to people who do not get the disease (N-d) Note: Odds are rarely used to measure disease frequency Using Table 1: 18 Rates or incidence rates Rates, like risks, are a measure of the frequency of occurrence of new cases of a disease (d) They relate the number of new cases to the person- time at risk (Y). The rate (R) is estimated by dividing the total number of events observed among all individuals, (d), by the sum of the individual observation times (Y) 19 Estimating population-time at risk 3 main methods; Method 1: Add up the time that each person is at risk – Most commonly used Method 2: Add up the population at risk during each time segment Method 3: Multiply the average size of the population at risk by the length of the time interval – Usually used for large populations – Multiply the average pop at the mid-point of the calendar period of interest (mid-period population) by the number of years of the study 20 Understanding person years I 5 21 Understanding person years IIa Time at risk (years) 30 Person 2 stayed in the study all 40 years and did not 40 develop the outcome 20 20 40 40 20 40 20 Person 10 dropped out of 30 the study at Year 30 20 30 40 10 10 40 40 40 10 Person 19 developed the 20 outcome at Year 10 Total: 560 person-years X developed disease; O loss to follow-up 22 Understanding person years IIb Alt Years 0-10= 20x10=200 py Years 11-20=17x10=170 py Years 21-30=11x10=110 py Years 31-40=8x10=80 py Total=200+170+110+80 = 560 py X developed disease; O loss to follow-up 23 Calculating the incidence rate § The numerator is the number of cases § The denominator is the total person-time § In our example above: Incidence rate = 8/560 = 0.014, or a rate of 14 cases per 1,000 person-years 24 Uses of incidence & prevalence Incidence Prevalence Generally used for Used for more permanent states, acutely acquired conditions or attributes of ill-health diseases Can be used to Important for assessing the public investigate causal health impact of a specific disease relationships within a community, & estimating what services are required including the costs & resources consumed as a result of the disorder Always requires a May or may not require a duration duration 25 Measures of association & impact (relative & absolute measures) 26 Relative measures Ratio of measures of disease frequency for the two groups of subjects – probability of developing the disease in the exposed individuals relative to those unexposed – estimate the size of an association between exposure & disease – indicate how much more likely people in an exposed group are to develop the disease than those in an unexposed group 3 relative measures – Risk ratio – Rate ratio – Odds ratio 27 Risk ratio Risk ratio=Risk in the exposed group Risk in the unexposed group Risk ratio=a/(a+b) c/(c+d) 28 Rate ratio Rate ratio = Incidence rate in the exposed group Incidence rate in the unexposed group 29 Odds ratio Odds ratio = Odds of disease in the exposed group Odds of disease in the unexposed group Odds ratio = a/b c/d Odds ratio = ad bc For a rare disease; Risk ratio ≈ odds ratio ≈ rate ratio 30 Measures of PH relevance: Absolute measures Difference between the measures of disease frequency in the two groups – i.e. absolute effect of exposure in those exposed compared with those unexposed Absolute measures indicate exactly what impact a particular disease or condition will have on a population – This has important implications for any public health prevention measures we may want to take 4 main absolute measures of public health relevance – Attributable (absolute) risk – Attributable risk per cent – Population attributable (absolute) risk – Population attributable fraction 31 The concept of attributable risk Background risk A B 32 The concept of attributable risk II Incidence in the Incidence not due to the non-exposed = exposure group (background exposure) Incidence not due to the Incidence in the Incidence due to exposed group = exposure + the exposure (background exposure) Alternatively, Incidence in the Incidence due to Incidence in the the exposure = exposed group - non-exposed group 33 Attributable (absolute) risk The excess risk caused by exposure in the exposed group – Assumes that the relationship between the exposure and disease is causal – The attributable risk is the excess risk (or rate) in the exposed due to exposure Attributable risk= risk in the exposed - risk in the unexposed 34 Attributable risk per cent The attributable risk can be expressed as the proportion of disease in the exposed group attributable to the exposure (the proportion of additional cases). This the attributable risk per cent 35 Population attributable (absolute) risk (PAR) What is the impact of an exposure on the whole population? – The excess rate in the population due to the exposure PAR=Incidence in the whole population- Incidence in the unexposed population Note: PAR is only applicable when the exposure is causal action 36 Population attributable fraction (PAF) Indicates the proportion of the disease in the population that could be prevented if the exposure to the risk factor could be eliminated and the entire population was unexposed – i.e. the proportion of cases that could be avoided if the exposure were removed from the population entirely. PAF=Population attributable risk Risk of disease in population 37 Measures of disease occurrence and exposure effect in analytic study designs 1Unless the sampling fraction is known for both cases and controls, i.e. unless the proportion of cases and proportion of controls sampled from the population is known 38 Sources of health data 39 Health statistics The health status of a population is assessed by the collection, analysis and interpretation of data about important events that serve as indicators of the health of the community – Deaths (mortality data) – Sickness (morbidity data) – Utilization of health services Such data is important for the planning, implementation and evaluation of health services 40 Types of health data Vital statistics – Records of vital events (births, deaths, marriages & divorces) obtained by registration Morbidity statistics – occurrence & severity of sickness in a community – may be obtained from health services Health service statistics – obtained from the operations of health services – 2 types: resources data & institutional records Data from other sectors – education (eg literacy rates) – public works (housing, water supply, sanitation) – agriculture (food production & distribution) – economic planning (poverty, economic indicators) 41 Sources of health data Source Examples Type of data Census Local, national Total count, age, sex dist Epidemiological surveys Questionnaires Sickness surveys Sickness, absence from work Physical examination Nutritional survey Anthropometric measurements Special investigations Serological, TB surveys Prevalence of HIV infection Medical institutions Outpatient clinics Heath centre Clinical records Special clinics ANC, STI, Diabetes clinics Attendance records, health profile, Inpatient services Gen & specialist hospitals Clinical, lab, autopsy data Data collected for other purposes Routine medical exam Sch entrants, employment Nutritional, immunization status, health profile Sickness absence records Schools, industry Early warning of epidemics 42 Discussion What is the differences between prevalence and incidence? Define the three measures of association State any two measures of Public Health relevance State any two sources of health data 43 References Celentano DD, Szklo M (2019). Gordis Epidemiology, 6th ed. Philadelphia: Elsevier (Saunders) Bailey L, Vardulaki K, Langham J, Chandramohan D (2005). Introduction to Epidemiology. Maidenhead: Open University Press Bonita R, Beaglehole R, Kjellstrom T (2006). Basic Epidemiology, 2nd ed. Geneva: WHO Hennekens CH, Buring JE (1987). Epidemiology in Medicine. Philadelphia:Lippincott Williams & Wilkins: Lucas AO, Gilles HM (2003). A Short Textbook of Public Health Medicine for the Tropics, 4th ed. London: Arnold 44 Thank you 45

Measures of Health & Disease PDF

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