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Lecture 3

HSCI 130 Lecture Notes - Lecture 3: Prevalence, Handedness, Relative Risk

Health Sciences
Course Code
HSCI 130
Robert Hogg

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Wednesday, January 17, 2018
HSCI 130: Measurement
Lecture #3
Population = a group of people with common characteristic like age, race, sex. There are two types of populations, based on
whether membership is permanent or transient
— Fixed population = membership is permanent and defined by an event e.g. atomic bomb survivors
— Dynamic population = membership is transient and defined by being in or out of “state” e.g. residents of the city of Vancouver
— Disease Frequency = want to quantify disease occurrence in a population. Measures of disease frequency should take into account:
— 1) the number of individuals affected with the disease
— 2) size of source population
— 3) Length of time the population was followed
— Epidemiology needs the number and characteristics of disease cases, of people with risk factors and of the population from which
the above people are derived
— Numbers of cases, or people with the risk factors compromise the numerator. The population from which they come is the
— Rate in epidemiology = Term given to the fraction,when numerator is divided by the denominator
A ratio = a ratio is one number in relation to another and a rate = is a ratio
— Prevalence (P) = the number of existing cases of disease / Number in total population (at a point during a period of time)
— e.g. City A has 7000 people with arthritis on Jan 1st. 1999, population of City A = 70,000 equation P= 7000 / 70,000
ANSWER = Prevalence of arthritis on January 1st = = 0.10 or 10%
— when using the equation for prevalence you must express the answer in decimal and percentage form, as well as include the time
*also make sure to clearly define your numerator and denominator*
— Prevalence = the number of existing cases of a disease at a point in time/ population at risk
— Numerator = Number of people who are ill, who have specific symptoms, or who have microbiological evidence of infection but
remain asymptomatic
— Denominator = Number of people in the population who are susceptible to the disease and/or who were exposed
Types of Prevalence: for prevalence :unlike incidence, you include the people who have the disease in the denominator
— Point prevalence = compromised all the cases of a disease that exist at a point in time
— Period prevalence = is al cases whether old, new or recurrent, arising over a defined period, say a year or two. The
denominator is the average population over the period.
— Lifetime prevalence = is the proportion of the population who have ever had the disease
Uses of Prevalence: 4 uses
— 1) Quantify the proportion of people with a disease (how many people are affected)
— 2) Estimate the probability that an individual will have the disease during a point in time
— 3) Project health care and other policy needs or issues
— 4) Estimate the costs associated with a particular disease
EXAMPLE: What is the prevalence of left-handedness (a potentially deadly infectious disease identified on SFU campus in
— Incidence = quantified number of new cases of disease that develop in a population at risk during a specified time period. There
are 3 key concepts:
— 1) New disease events or for diseases that can occur more than once, usually first occurrence of disease
— 2) Population at risk (candidate population)—can’t have disease already, should have relevant organs
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Wednesday, January 17, 2018
— 3) Time must pass for a person to move from health to disease
Incidence rate = measure of the rate of development of a disease in a population. It is calculated by the equation : counting the
number of people who develop the disease / number of person-years when people were at risk for developing the disease. It
measure how quickly people are developing the disease. Incident rates are true rates—the incidence rate can go up or down. The
Denominator represents window of time people are at risk of disease—not number of people at risk at the beginning.* It is reported in
a unit of person-years of follow up e.g. 28 cases per 1,000 person years of observation*
Incidence measures: 1) Cumulative incidence (CI) and 2) Incidence Rate (IR)
— 1) Cumulative Incidence (CI) = calculated by the equation: CI = number of new cases of disease / number of
candidate population over a specified period of time .
— Cumulative incidence estimated the probability or risk that a person will develop disease DURING A SPECIFIED
TIME. Note that the candidate population is compromised of people who are “at risk” of getting the disease. It is used mainly for
fixed populations because it assumes that everyone is followed for the entire time period. e.g. CI of SIDS (sudden infant death
syndrome), population = 1,000 live births, Cases of SIDS = 10, —-> ANSWER = CI = 10/1,000 = 0.01 or 1% over one year (all
births = “at risk”)
4 Problems with CI:
— 1) CI calculation assumes that you have followed the entire population for the entire follow-up period. For example, it
assumes you have followed all of the live births for one year or until SIDS occurred.
— 2) Often you cat follow everyone for entire time period
— 3) In a dynamic population, individuals enter population over time, become lost, etc., so length of follow up is not uniform
for everyone or all
— 4) incidence rates do not make assumption of complete follow up
— 2) Incidence Rate (IR) = The number of new cases of disease per unit of person time at risk. The equation is: # of new
cases of disease during a time period / # of person-years when people were at risk for developing the disease. *The numerator
does not differ between the two types of incidence measures BUT the DENOMINATOR does not differ *
* The time unit for person-time = year, month or day* * Person-time - person-year, person-month, person-day*
— EXAMPLE: Followed 1, 762 women —-> 30,324 Person-Years, Average of 17 years of follow-up per woman, Ascertained 61
cases of breast cancer——> ANSWER: IR = 61/30,324 Person years = o.00201/years —-> = 201/100,000 person/years
— Incidence = is a measure of the risk of developing disease within a specified period of time.
— Cumulative incidence = the number of new cases within a specified time period dividing by the size of the population initially at
risk The equation is: CI = # of new cases of disease within a time period / # of people at risk in the population at the beginning
of the specified tim period
— NOTE: A precede definition of th population at risk should be included. The definition of populations at risk should include:
1) people who currently have the disease
— 2) people who cannot develop the disease (age, sex, immune, etc
— EXAMPLE OF CI of chicken Pox over the school years. 1000 student enrolled at Lucy’s
elementary school. On day 1 of school: 50 kids are absent due to chicken pox, 200 report having
previously had chicken pox, 150 report having pervasively received the chicken pox vaccine
1) What is the cumulative incidence of chicken pox in Lucy’s school over the course of the entire
year? ANSWER —-> 155/600 = 0.258 or 25.8%
2) What is the cumulative incidence of chicken pox in Lucy’s school over the course of the first
semester (Sept-Dec)? ANSWER —-> 95/ 600 = 0.158 or 15.8%
3) What is the cumulative incidence of Chicken pox in Lucy’s school over the course of the second
semester (Jan-June)? ANSWER —-> 60 /505 = 0.119 or 11.9%
New Cases of Chicken Pox
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