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Introduction to Epidemiology-2

Introduction to Epidemiology-2. Specific learning objectives. To understand the concept of case and study population To know the common measures of occurrence used in epidemiology and understand their applications To introduce the measures of effect: Risk Ratio. Concept of case definition.

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Introduction to Epidemiology-2

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  1. Introduction to Epidemiology-2

  2. Specific learning objectives • To understand the concept of case and study population • To know the common measures of occurrence used in epidemiology and understand their applications • To introduce the measures of effect: Risk Ratio

  3. Concept of case definition • Before we quantify the frequency of occurrence, we must define it clearly • It is a set of standardized criteria used to identify cases • Thus it must have: • Methods to identify a case • Boundaries of a case • The unit of analysis

  4. Method to identify a case • Uniform procedure or instruments have to be used to identify the case • For eg: Compare breast cancer in population X and Y • If one uses clinical examination and the other used mammography/biopsy to detect the case, it cannot be compared

  5. Boundaries of case • There has to be clear dividing line between cases and non-cases • This is unlike clinical medicine • Eg: Breast cancer cases are being surveyed • Pre-invasive cancer is case or non-case will have to be distinctly defined and adhered to during the study • In case of continuous variables, this means defining a cut-off

  6. Unit of analysis • Unit of analysis used for case definition will affect the estimation of occurrence • Eg: Breast cancer • Following possible units of measurements: • Women with breast cancer • A breast with cancer • Number of malignant tumors in a breast

  7. Unit of analysis Case definition of acute diarrhea in primary school: An episode in which a child reports at least 3 loose stools per day. It should last for at least 48 hours but less than 14 days Any child who reports at least one episode of at least 3 loose stools per day lasting for at least 48 hours but less than 14 days Identify the difference

  8. Developing case definitions • Factors that need to be considered: • The question that we want to answer • The resources available • The ethical issues

  9. Measures of disease occurrence Following concepts need attention • Types of fractions used in describing disease occurrence • Prevalence (P) • Incidence (I) • Relation between P and I

  10. Types of Fractions Three general fractions from mathematics are used: • Ratio • Proportion • Rate

  11. Ratio • A fraction where there is no specific relationship between numerator and denominator • Relation in size between two random quantities • Numerator is not a part of denominator • Eg: Sex ratio is 940 in India • It means there are 940 females for 1000 males • Eg: Doctor:Population ratio

  12. Proportion • It is a ratio where numerator is always part of denominator • Proportion indicates part of the whole • It is usually expressed as a percentage • Eg: Fetal death rate: fetal deaths/all births • All births include live births and fetal deaths

  13. Rate • A ratio where there is a distinct relationship between numerator and denominator and most importantly measure of time as an intrinsic part of the denominator • It measures the occurrence of some particular event in a population during a given time period • It is a statement of risk of a condition

  14. Elements in a rate: numerator, denominator, time specification, and a multiplier • The multiplier can be 1000 or 10,000 or more • Eg: Crude death rate in a year No of deaths in one year X 1000 Mid-year population • Birth rate, breast cancer rate in Indian women

  15. Measures of disease occurrence • It is important to differentiate and be specific about defining numerator or denominator • Disease/health event or individuals/persons • Types of measures of occurrence: - Prevalence - Incidence

  16. Prevalence • It quantifies the proportion of individuals in a population • Who have an attribute or disease at a particular time (or a period) • It may be new and pre-existing disease or attribute • Divided by the population at risk at this point in time or midway of the period

  17. P = Number of existing cases of a disease Total population at a given point in time Types: • Point prevalence • Period prevalence

  18. Point prevalence • Proportion of individuals in a specified population at risk who have the health or disease attribute of interest at a given point in time • The point refers to specific point in calendar • May be day, days, weeks depending on the time it takes to examine the population sample

  19. Example: Identify two measures of prevalence: All adults of more than 50 years (n = 331) residing in the sub-center Kondur, were examined for lenticular opacity and visual acuity during November and December 2010 There were 24.8% persons who had cataract  Only 13% of the persons with cataract were operated at the time of interview

  20. Period prevalence • Proportion of individuals in a specified population at risk who have the disease/health event of interest over a specified period of time/time interval • Eg: Annual prevalence rate of diabetes • This will include cases already existing and those that arise during that year

  21. Incidence • Incidence quantifies the number of new events or cases of disease that develop a disease during a specified period of time • Thus only new cases • During a given period • In a specified population or population at risk • It can even be spells/episodes of a disease instead of persons

  22. Types of incidence • Incidence proportion or risk or attack rate • Incidence rate or person-time rate

  23. Incidence proportion or risk or attack rate • It is the proportion of initially disease free population that develops disease/attribute during a specified period of time • It is a proportion because persons in the numerator are included in the denominator

  24. Incidence Proportion calculation Example: In the study of diabetics, 100 of the 189 diabetics died during 13 year follow-up Calculate the incidence proportion/risk of death (100/189) X 100 = 52.9%

  25. Example: In an outbreak of gastroenteritis (GE) among attendees of a picnic of 125 persons, 99 persons ate potato salad. Thirty developed GE. Calculate the risk of illness among persons who ate potato salad Numerator: 30 Denominator: 99 Risk = (30/99) X 100 = 30.3%

  26. Uses of Incidence proportion/risk • It is a measure of risk of disease or the probability of developing the disease during the specified period of time Importance of appropriate denominator: • The denominator should be limited to ‘population at risk’ • Eg: if risk is to be calculated for ovarian cancer, the denominator should be restricted to women

  27. Incidence rate or person-time rate • As the word rate suggests: time is part of denominator • The numerator is same as incidence risk • The denominator is different • It is the sum of the time each person was observed, totaled for all persons • Total time the population was at risk of and being watched for disease

  28. Example The diabetes follow-up study included 218 diabetic women and 3,823 nondiabetic women. By the end of the study, 72 of the diabetic women and 511 of the nondiabetic women had died. The diabetic women were observed for a total of 1,862 person years; the nondiabetic women were observed for a total of 36,653 person years. Calculate the incidence rates of death for the diabetic and non-diabetic women.

  29. For diabetic women, numerator = 72 and denominator = 1,862 • Person-time rate = 72 ⁄ 1,862= 0.0386 deaths per person-year= 38.6 deaths per 1,000 person-years

  30. For nondiabetic women, numerator = 511 and denominator = 36,653 • Person-time rate = 511 ⁄ 36,653 = 0.0139 deaths per person-year= 13.9 deaths per 1,000 person-years

  31. Answers to in-class exercise 1- A; denominator is size of population at start of study, numerator is number of deaths among that population. 2 - B; denominator is person-years contributed by participants, numerator is number of death among that population. 3 - C; numerator is all existing cases. 4 - A; denominator is size of population at risk, numerator is number of new cases among that population. 5 - B; denominator is mid-year population, numerator is number of new cases among that population. 6 - C; numerator is total number with attribute. 7 - D; this is a ratio (heart disease : smokers)

  32. Relation between P & I Prevalence = Incidence X Duration of each case Scenario discussion: - If all the other factors were kept constant, what will happen to the prevalence of HIV in a specified population following the introduction of a new drug regimen that prolongs life but does not cure the disease

  33. The prevalence of HIV will increase. P = I X D Incidence is kept constant (difficult in real life situation) Duration increases Hence P increases

  34. - If all other factors are kept constant, what would happen to the prevalence of asthma in a specified population following the introduction of a new treatment regimen that cures the disease?

  35. Prevalence of asthma would decrease • Some patients are cured • So the average (D) duration of disease would decrease • P = I X D • Hence P will decrease

  36. - If all other factors were kept constant, what would happen to the prevalence of lung cancer in a specified population if the incidence decreases after a ban on tobacco of a prolonged nature

  37. Prevalence of lung cancer would decrease • All other factors are constant • Fewer new cases develop • So incidence reduces • P = I X D

  38. Measures of effect and impact

  39. Measure of effect(measure of association) • The key to epidemiologic analysis is comparison • Do you observe: • Incidence of a disease seems high in a community • Wonder whether it is truly high than other community or not • How to resolve this?............Compare

  40. Recall the analytical epidemiology • Descriptive epidemiology: just describe the exposure and outcome without an attempt to look for association • Analytical epidemiology: We try and look for association between exposure and outcome • Such exposures can be harmful or protective

  41. Measures of effect/association quantifies the relationship between exposure and disease among two groups. • These are measured in the form of: • Relative risk (Risk ratio) • Rate ratio • Odds ratio

  42. Exposures • Exposure is used loosely to mean not only exposure to foods, mosquitoes, a partner with a sexually transmissible disease, or a toxic waste dump • But also inherent characteristics of persons (for example, age, race, sex) • Biologic characteristics (immune status) • Acquired characteristics (marital status) • Activities (occupation, leisure activities) • Conditions under which they live (socioeconomic status or access to medical care).

  43. Risk ratio or relative risk • Compares the risk of a health event (disease, injury, risk factor, or death) among one group with the risk among another group • Done by dividing the risk (incidence risk) in group 1 by the risk (incidence risk) in group 2 where one has the suspected factor and other does not • The group with exposure/suspected factor: Exposed group • The other: Unexposed group

  44. Relative Risk/Risk Ratio(RR) • The formula for risk ratio (RR) is = Risk of disease (incidence risk) in group of primary interest  Risk of disease (incidence risk) in comparison group • RR of 1 indicates identical risk • RR > 1 indicates greater risk • RR< 1 indicates decreased risk (protection due to exposure)

  45. Example • In an outbreak of tuberculosis among prison inmates in South Carolina in 1999, 28 of 157 inmates residing on the East wing of the dormitory developed tuberculosis, compared with 4 of 137 inmates residing on the West wing. • Summarize this data in 2 X 2 table as per your back ground reading material

  46. Two by two table Data Source: McLaughlin SI, Spradling P, Drociuk D, Ridzon R, Pozsik CJ, Onorato I. Extensive transmission of Mycobacterium tuberculosis among congregated, HIV-infected prison inmates in South Carolina, United States. Int J Tuberc Lung Dis 2003;7:665–672.

  47. Risk ratio • RR = Incidence risk in exposed (Ie) Incidence risk in unexposed (Iue) • Risk of tuberculosis among East wing residents = 28 ⁄ 157 = 0.178 = 17.8% • Risk of tuberculosis among West wing residents = 4 ⁄ 137 = 0.029 = 2.9% • Risk ratio = 17.8 ⁄ 2.9 = 6.1

  48. Example In an outbreak of varicella (chickenpox) in Oregon in 2002, varicella was diagnosed in 18 of 152 vaccinated children compared with 3 of 7 unvaccinated children. First design a 2 X 2 table Calculate the risk ratio.

  49. Data Source: Tugwell BD, Lee LE, Gillette H, Lorber EM, Hedberg K, Cieslak PR. Chickenpox outbreak in a highly vaccinated school population. Pediatrics 2004 Mar;113(3 Pt 1):455–459.

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