Epidemiologic_measures2__5689ccb6.ppt in
- 1. Epidemiologic Measures Afshin Ostovar Bushehr University of Medical Sciences Bushehr, 2012 01/28/25 1
- 5. Frequency Measures Ratios Proportions Rates 01/28/25 5
- 7. Ratio 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 01/28/25 7
- 8. Proportion 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 01/28/25 8
- 9. Rate 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: 01/28/25 9
- 10. Three important aspects of this formula: The persons in the denominator must reflect the population from which the cases in the numerator arose. The counts in the numerator and denominator should cover the same time period. 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. 01/28/25 10
- 11. 01/28/25 11
- 12. 01/28/25 12
- 13. 01/28/25 13
- 14. a. Point prevalence on October 1, 1990? b. Period prevalence, October 1, 1990 to September 30, 1991? 01/28/25 14
- 15. Attack Rate 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. The attack rate is usually expressed as a percent, so 10n equals 100. 01/28/25 15
- 16. Attack Rate Example Of 75 persons who attended a church picnic, 46 subsequently developed gastroenteritis. 01/28/25 16
- 17. Secondary Attack Rate frequency of new cases of a disease among the contacts of known cases. 01/28/25 17
- 18. 01/28/25 18
- 20. Measures of Association Risk Ratio (Relative Risk) Rate Ratio (Relative Rate) Odds Ratio (cross-product ratio) 01/28/25 20
- 21. Risk Ratio (Relative Risk) A risk ratio, or relative risk, compares the risk of some health-related event such as disease or death in two groups. 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). Often, you will see the group of primary interest labeled the “exposed” group, and the comparison group labeled the “unexposed” group. 01/28/25 21
- 22. Risk Ratio (Relative Risk) 01/28/25 22
- 23. Risk Ratio (Relative Risk) 01/28/25 23
- 24. Rate Ratio A rate ratio compares two groups in terms of incidence rates, person-time rates, or mortality rates. Like the risk ratio, the two groups are typically differentiated by demographic factors or by exposure to a suspected causative agent. 01/28/25 24
- 27. Odds Ratio (cross-product ratio) a = number of persons with disease and with exposure of interest b = number of persons without disease, but with exposure of interest c = number of persons with disease, but without exposure of interest d = number of persons without disease and without exposure of interest a + c = total number of persons with disease (“cases”) b + d = total number of persons without disease (“controls”) 01/28/25 27
- 28. Odds Ratio (cross-product ratio) 01/28/25 28
- 29. Attributable Proportion The attributable proportion, also known as the attributable risk percent, is a measure of the public health impact of a causative factor. 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). 01/28/25 29
- 33. 01/28/25 33
- 36. Direct Standardization, Cont. HTN Rates in Two Phases of MONICA Study, Bushehr 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% 01/28/25 36
- 40. 01/28/25 40
- 42. Indirect Standardization Observed / Expected Occupational Studies Standard Mortality Ratio 01/28/25 42