Epidemiologic Measures Afshin Ostovar Bushehr University of Medical

Epidemiologic Measures Afshin Ostovar Bushehr University of Medical Sciences Bushehr, 2012 11/9/2020 1

Frequency Distributions 11/9/2020 2

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frequency distribution 11/9/2020 4

Frequency Measures l l l 11/9/2020 Ratios Proportions Rates 5

Formula 11/9/2020 6

Ratio l In a ratio, the values of x and y may be completely independent, or x may be included in y. Exp: Male/Female Male/All 11/9/2020 7

Proportion l A proportion, the second type of frequency measure used with dichotomous variables, is a ratio in which x is included in y. Exp: Male/Female Male/All 11/9/2020 8

Rate l 11/9/2020 Rate, is often a proportion, with an added dimension: it measures the occurrence of an event in a population over time. The basic formula for a rate is as follows: 9

Three important aspects of this formula: l The persons in the denominator must reflect the population from which the cases in the numerator arose. l The counts in the numerator and denominator should cover the same time period. l In theory, the persons in the denominator must be “at risk” for the event, that is, it should have been possible for them to experience the event. 11/9/2020 10

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a. Point prevalence on October 1, 1990? b. Period prevalence, October 1, 1990 to September 30, 1991? 11/9/2020 14

Attack Rate l An attack rate is a variant of an incidence rate, applied to a narrowly defined population observed for a limited time, such as during an epidemic. l The attack rate is usually expressed as a percent, so 10 n equals 100. 11/9/2020 15

Attack Rate Example Of 75 persons who attended a church picnic, 46 subsequently developed gastroenteritis. 11/9/2020 16

Secondary Attack Rate l frequency of new cases of a disease among the contacts of known cases. 11/9/2020 17

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Person-time Rate 11/9/2020 19

Measures of Association l Risk Ratio (Relative Risk) l Rate Ratio (Relative Rate) l Odds Ratio (cross-product ratio) 11/9/2020 20

Risk Ratio (Relative Risk) l A risk ratio, or relative risk, compares the risk of some health-related event such as disease or death in two groups. l The two groups are typically differentiated by demographic factors such as sex (e. g. , males versus females) or by exposure to a suspected risk factor (e. g. , consumption of potato salad or not). l Often, you will see the group of primary interest labeled the “exposed” group, and the comparison group labeled the “unexposed” group. 11/9/2020 21

Risk Ratio (Relative Risk) 11/9/2020 22

Risk Ratio (Relative Risk) 11/9/2020 23

Rate Ratio l A rate ratio compares two groups in terms of incidence rates, person-time rates, or mortality rates. l Like the risk ratio, the two groups are typically differentiated by demographic factors or by exposure to a suspected causative agent. 11/9/2020 24

Rate Ratio 11/9/2020 25

Rate Ratio 11/9/2020 26

Odds Ratio (cross-product ratio) l a = number of persons with disease and with exposure of interest l b = number of persons without disease, but with exposure of interest l c = number of persons with disease, but without exposure of interest l d = number of persons without disease and without exposure of interest l a + c = total number of persons with disease (“cases”) l 11/9/2020 b+d= total number of persons without disease (“controls”) 27

Odds Ratio (cross-product ratio) 11/9/2020 28

Attributable Proportion l The attributable proportion, also known as the attributable risk percent, is a measure of the public health impact of a causative factor. l Thus, the attributable proportion is the proportion of disease in an exposed group attributable to the exposure. It represents the expected reduction in disease if the exposure could be removed (or never existed). 11/9/2020 29

Attributable Proportion 11/9/2020 30

Attributable Proportion 11/9/2020 31

Mortality Rates 11/9/2020 32

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Natality Frequency Measures 11/9/2020 34

Direct Standardization 0. 365 0. 362 0. 36 HTN Rate 0. 355 0. 345 0. 34 0. 335 0. 3341 0. 325 0. 32 Phase 1 Phase 2 HTN Rates in Two Phases of MONICA Study, Bushehr 11/9/2020 35

Direct Standardization, Cont. HTN Rates in Two Phases of MONICA Study, Bushehr 11/9/2020 Age Group (Years) Phase 1 Phase 2 25 -34 15. 2% 10. 4% 35 -44 29. 4% 20. 3% 45 -54 50. 7% 43. 0% 55 -66 64. 8% 56. 8% >67 85. 7% 70. 8% 36

Direct Standardization, Cont. Hypertension rate change in different age groups in the first and second phases 90. 0% 80. 0% 70. 0% 60. 0% 25 -34 50. 0% 35 -44 40. 0% 45 -54 30. 0% 55 -66 >67 20. 0% 10. 0% Phase 1 11/9/2020 Phase 2 37

Direct Standardization 0. 365 0. 362 0. 36 HTN Rate 0. 355 0. 345 0. 34 0. 335 0. 3341 0. 325 0. 32 Phase 1 Phase 2 HTN Rates in Two Phases of MONICA Study, Bushehr 11/9/2020 38

Direct Standardization Age Distribution in Two Phases of MONICA Study, Bushehr 1600 1400 1200 1000 Phase 1 800 Phase 2 600 400 200 0 25 -34 11/9/2020 35 -44 45 -54 55 -66 >67 39

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Direct Standardization 0. 38 0. 365458 0. 34 0. 32 0. 303358 0. 26 0. 24 0. 22 0. 2 Phase 1 Phase 2 Directly Age Adjusted HTN Rates in Two Phases of MONICA Study, Bushehr 11/9/2020 41

Indirect Standardization l l l 11/9/2020 Observed / Expected Occupational Studies Standard Mortality Ratio 42