Epidemiology Lesson 5
Epidemiology Lesson 5 HS 370
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This 6 page Class Notes was uploaded by Cindy Cannon on Monday February 22, 2016. The Class Notes belongs to HS 370 at Brigham Young University - Idaho taught by Watson, Tyler A. in Fall 2016. Since its upload, it has received 16 views. For similar materials see Epidemiology in Nursing and Health Sciences at Brigham Young University - Idaho.
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Date Created: 02/22/16
Epidemiology Lesson 5 Measure of Risk: Part 1 In order for epidemiologists to compare rates from different studies, locations and diseases, it is important to understand the specific diseases calculations, how they are completed, and what they mean o Incidence= New case of diseases over a period of time/Population were the disease occurred that are at risk o Prevalence= Cases of disease at a POINT (an actual date) in time/ Total population at midyear that are at risk o Crude Death Rate= Total deaths/ Population where deaths occurred o Proportional Mortality Rate= Total deaths FROM disease X/All deaths during the period o Case Fatality Rate or Proportion=Deaths from X/All cases of X o Cause Specific Mortality= Deaths from X/ Total population where deaths occurred Rules You can never calculate a proportion as less than a percent (%) (Per 100) You never move the decimal so that the proportion is MORE than per million After converting a proportion to a decimal value, more the decimal place to the right, until you get past the first digit larger than 0. Retain the next 2 digits as significant digits o To correctly, identify the multiplier you count the number of times you moved the decimal place and add that many 0 behind it Ex o 39/1,875=0.2026=2.03/100 or 2.03% o 38/12,540=0.00303=3.03/1,000 o 38/279,583=0.0001359=1.36/10,000 Frequency Measure Frequency measures compare one part of the distribution to another part of the distribution, or to the entire distribution. 3 common frequency measures o Ratios: the relative magnitude of 2 quantities or a comparison of any 2 values Method: Number or rate of events, items, persons, etc. in 1 group/Number or rate of events, items person, etc. in another group Epidemiology Lesson 5 1 After the numerator is divided by the denominator the result is often expressed as the result “to one” or written as the result “: 1” Ex: in a study 3,151 mean were non-diabetic and 189 were diabetic. Calculate the ratio of non-diabetic to diabetic men o Ratio=3,151/189X1=167:1 o A commonly used epidemiologic ratio: death-to-case ratio o Proportion: the comparison of a part to a whole and the numerator is included in the denominator Method: (Number of persons or events with a particular characteristic/Total number of persons or events, of which the numerator is a subset) X 10^n 10^n is usually 100 (n=2) and is often expressed as percentage o Ex: 189 diabetic men and 3,340 total number of men Proportion= (189/3,340) X100=5.66% A specific type of epidemiology proportion: proportionate mortality o Rate: a measure of frequency with which an even in a defined population over a specific period of time Incidence Rate: a sense of speed with which disease occurs in a population Attach Rate: the proportion of the population that develops illness during an outbreak Also called an Incidence proportion Prevalence Rate: the proportion of the population that has a health condition at a point in time Case-fatality Rate: the proportion of persons with the disease who die from it Morbidity Frequency Measures Morbidity: any departure subjective or objective, from a state of physiological or psychological Measures of morbidity frequency characterize the number of persons in a population who become ill (incidence) or are ill at a given time (prevalence) o Incidence: the occurrence of new cases of disease or injury in a population over a specified period of time Incidence Proportion (Risk): the proportion of an initially disease-free population that develops a disease becomes injured or dies during a specified (usually limited) period of time Epidemiology Lesson 5 2 Method number of new cases of disease or injury during specified period/Stage of population at start of a period Ex: 100 if the 189 diabetic men died during a 13-year follow up period Risk= (100/189) X 100=5.9% In the outbreak setting, the term attack rate is often used as a synonym for risk. It is the risk of getting the disease during a specified period, such as during an outbreak Overall Attach Rate: the total number of new cases divided by the total population Food Specific Attach Rate: the number of persons who ate a specific food and became ill divided by the total number of persons who ate that food. Secondary Attach Rate: the difference between community transmission of illness vs. transmission of illness in a household, barracks, or other closed population. Incidence Rate or Person-Time Rate: measure of incidence that incorporates time directly into the denominator It’s generally calculated from a long-term cohort follow-up study, wherein enrollees are followed over time and the occurrence of new cases of disease is documented Method: (Number of new cases of disease or injury during specified period/ Time each person was observed totaled for all persons) Ex: Annual study of 2,100 women for 4 years for heart disease Year: 1 2 3 4 Cases: 0 1 7 8 # Lost Contact: 100 99 793 392 Numerator (Number of Cases)=0+1+7+8=16 Denominator (Person-Years of Observation)=(2,000+(½ X 0)+(½ X 100)) +(1,900 +(½ X 1)+(½ X 99))+(1,100+(½ X 7)+(½ X 793))+(700+(½ X 8)+(½ X 392))=6,400 o Person-Time Rate: 16/6,400=0.0025=2.5 cases per 1,000 person-year Epidemiology Lesson 5 3 o Prevalence Rate: the proportion of persons in a population who have a particular disease or attribute at a specified point in time or over a specified period of time Point Prevalence: the proportion of persons with a particular or attribute on a particular date Period Prevalence: the proportion of persons with a particular disease or attribute during anytime during the interval o Method for Prevalence of Disease: (All new and pre-existing cases during a given time period/Population during the same time period) X 10^n o Method for Prevalence of an Attribute: (Persons having a particular attribute during an given time period/Population during the same time period) X 10^n o Difference between Prevalence and Incidence Prevalence refers to proportion of persons who have a condition at a or during a particular time period, whereas incidence refers to the proportion or rate of persons who develop a condition during a particular time period. The numerator of an incidence proportion or rate consists only of persons whose illness began during a specified interval, and the numerator for prevalence includes all persons ill from a specified cause during the specific interval regardless of when illness began Mortality Frequency Measures Mortality Rate: measure of the frequency of occurrence of death in a defined population during a specified interval o Method: (Deaths occurring during a given time period/Size of the population among which the deaths occurred) X 10^n o Crude Mortality Rate: mortality rate from all causes of death for a population o Cause-Specific Mortality Rate: mortality rate from a specified cause for a population o Age-Specific Mortality Rate: mortality rate limited to a particular age group o Infant Mortality Rate Method: (Number of deaths among children< 1 year of age, reported during a given time period/Number of live births reported during the same time period) X 1,000 Neonatal Mortality Rate: mortality rate of children under 28 days of age during a given time period Epidemiology Lesson 5 4 Post neonatal Mortality Rate: mortality rate of children form 28 days up to, but not including, 1 year of age during a given time period The denominator of both of these is the number of live births during the same time period o Maternal Mortality Rate: mortality ratio used to measure the (deaths during a given time period among women while pregnant or within 42 days of termination/Number of live births during the same time period) o Sex-specific Mortality Rate: (Mortality rate among either males or females/ Population of that same gender) o Race-Specific Mortality Rate: (Mortality rate related to a specified racial group/ Population of that same racial group) Case-Fatality Rate (Ratio): the proportion of persons with a particular condition (causes who die from that condition o Method: (Number of cause-specific deaths among the incident cases/Number of incident cases) X 10^n Proportional Mortality: the proportion of deaths in a specified population over a period of time attributable to different causes o Method: (Deaths caused by a particular cause/ Deaths from all causes) X 100 Years of Potential Life Lost (YPLL): one measure of the impact of premature mortality on a population o Method for Calculating YPLL form a Line Listing Step 1: decide on end point (65 years, average life expectancy or other) Step 2: exclude records of all persons who died at or after end point Step 3: for each person who died before the end point, calculate that person’s YPLL by subtracting the age at death from the end point Step 4: sum the individual YPLLs o Method of Calculating YPLL from a Frequency Step 1: ensure that age groups break at the identified endpoint (e.g. 65 years). Eliminate all age groups elder than the endpoint Step 2: for each age group younger than the endpoint, identify the midpoint of the age group ((Age group’s youngest age in years + oldest age+1)/2) Step 3: for each age group younger than the endpoint, identity that age group’s YPLL by subtracting the midpoint from the endpoint Epidemiology Lesson 5 5 Step 4: calculate age-specific YPLL by multiplying the age group’s YPLL times the number of persons in that age group Step 5: sum the age-specific YPLL’s o YPLL Rate: years of potential life lost per 1,000 populations below the end-point age, such as 65 years Method: (Years of potential life lost/ Population under age 65 years) X 10^n Epidemiology Lesson 5 6
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