Measures of Disease Occurrence - Health - Lecture Slides, Slides of Public Health

Download Measures of Disease Occurrence - Health - Lecture Slides and more Slides Public Health in PDF only on Docsity! Measures of Disease Occurrence docsity.com Learning Objectives: 1. Understand counts, ratios, proportions, and rates. 2. Define, calculate, and interpret incidence. 3. Understand the use of person-time denominators. 4. Distinguish between cumulative incidence and incidence rate. 5. Define, calculate, and interpret prevalence. 6. Distinguish between point and period prevalence. docsity.com • Simplest/most frequently performed measure in epidemiology • Refers to the number of cases of a disease or other health phenomenon being studied i.e. cases of influenza in Allegheny county in January, 2002 i.e. Number of persons involuntarily referred for psychiatric crisis intervention • Useful for allocation of health resources • Limited usefulness for epidemiologic purposes without knowing size of the source population Counts docsity.com Counts – Limited Interpretation New Cases Reporting Location of Disease Period Population City A 20 1998 100 City B 100 1998 1000 Annual Rate of Occurrence City A: 20 / 100 = 1 / 5 City B: 100 / 1000 = 1 / 10 docsity.com l Persons included in the numerator are always included in the denominator: A Proportion: -------- A + B l Indicates the magnitude of a part, related to the total. l In epidemiology, tells us the fraction of the population that is affected. Proportions docsity.com l Like a proportion, is a fraction, BUT without a specified relationship between the numerator and denominator l Example: Occurrence of Major Depression Female cases = 240 240 ------------------------ = ---- 2:1 female to male Male cases = 120 120 Ratios docsity.com l A ratio in which TIME forms part of the denominator l Epidemiologic rates contain the following elements: • disease frequency (in the numerator) • unit size of population • time period during which an event occurs Rates docsity.com Calculate crude annual death rate in the US: Annual death count Crude death rate = ----------------------- x 1,000 Reference population (during midpoint of year) Death count in U.S. during 1990:2,148,463 U.S. population on June 30, 1990: 248,709,873 2,148,463 Crude death rate = -------------- x 1,000 = 8.64 per 1,000 248,709,873 Rates – Example docsity.com Incidence The development of new cases of a disease that occur during a specified period of time in previously disease-free or condition-free (“at risk”) individuals. docsity.com Incidence Incidence quantifies the “development” of disease --- Most fundamental measure of disease frequency and leads to the development of the concept of risk (i.e transition from non-diseased to diseased state) - Cumulative incidence (CI) (“Incidence proportion”) - Incidence rate (IR) (“Incidence density”) docsity.com Cumulative Incidence (CI) PROPORTION of individuals who become diseased during a specified period of time (e.g. all new cases during 1998) Range: 0 to 1.0 Also referred to as “incidence proportion.” docsity.com Cumulative Incidence (CI) l Keep in mind that over any appreciable period of time, it is usually technically impossible to measure risk. l This is because if a population is followed over a period of time, some people in the population will die from causes other than the outcome under study l The phenomenon of being removed from a study through death from other causes is referred to as ”competing risks”. docsity.com Incidence Rate (IR) No. new cases of disease during a given period IR = ----------------------------------------------------------- Total “person-time” of observation Range = 0 to Infinity Since the number of cases is divided by a measure of time of observation, rather than people, this helps address the problem of competing risks. docsity.com Incidence Rate (IR) When we observe a group of individuals for a period of time in order to ascertain the DEVELOPMENT of an event…. - The actual time each individual is observed will most likely vary. What is person time? docsity.com Each subject contributes a specific person-time of observation (days, months, years) to the denominator Person Follow-up Time on Study Person Yrs. 1 <-------------------------------------> 2 2 <--------------------------------------D 2 3 <-----------------WD 1 4 <-------------------------------------------------------> 3 5 <-------------------------------------> 2 1995 1996 1997 1998 Jan. Jan. Jan. Jan. Person-Time docsity.com Person-Time Person Follow-up Time on Study Person Yrs. 1 <-------------------------------------> 2 2 <--------------------------------------D 2 3 <-----------------WD 1 4 <-------------------------------------------------------> 3 5 <-------------------------------------> 2 1995 1996 1997 1998 Jan. Jan. Jan. Jan. Study Period: 3 Years Study Participants: 5 Person Years of Observation: 10 Average Duration of Follow-Up: 2.0 Years docsity.com No. new cases of disease during a given period IR = ------------------------------------------------------------ Total “person-time” of observation So, 1 case IR = ----------- = 1 case per 10 years follow-up 10 years Whereas, 1 case CI = ------------ = 0.20 = 20.0% 5 persons Incidence Rate (IR) docsity.com Discussion Question 3 Consider the following: McDonald’s shooting lasting 1/2 hour with 50 patrons in the restaurant. 29 survivors: at risk period of 1/2 hr = 14.5 person hrs. 21 deaths: at risk period of avg. 1/4 hr = 5.25 person hrs. =21 deaths / 20 person hours This translates to 919,800,000 / 100,000 person years Therefore, as time increases, IR approaches infinity. docsity.com Incidence Rate (IR) NOTE: The selection of the time unit for the denominator is arbitrary, and is not directly interpretable: Example: 100 cases / person year can also be expressed as: 10,000 cases / person century 8.33 cases / person month 1.92 cases / person week 0.27 cases / person day docsity.com Incidence Rate (IR) l Incidence rate: - Incidence density - Force of morbidity l Measure of the instantaneous rate of development of disease in a population docsity.com