HSCI 307 Lecture Notes - Lecture 10: Relative Risk, Attributable Risk, Cohort Study

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Published on 24 Jan 2018
School
Simon Fraser University
Department
Health Sciences
Course
HSCI 307
Professor
Chapter 10 Measures of Frequency, Effect, and Outcomes
Measures of Frequency incidence and prevalence
Prevalence rate
The proportion of people at risk who have a specific condition at a given time
Prevalence rate = # of people with the disorder / # of people at risk in a given interval
Time 2, 8/100
Point prevalence vs. Period prevalence
Lifetime prevalence counts the # of people who have ever had the disorder at any time
in their lives
Lifetime prevalence can never decrease as people get older; it can only increase or
remain the same
Because once a person has a disorder, he or she should have it for life
Incidence rate
Looks at only the new cases
Some people already had the condition at the start of the interval (time 2, A B F I
K L)
Others developed it during the time frame (time 2, D G)
Incidence rate = # or new cases / # or people at risk in a given interval
Time 2, 2/ 94 (exclude A B F I K L)
All incident cases are also prevalent ones, but not prevalent cases are incident ones
Incidence density
D developed; C censored, the study ended before the people developed disorder, and
we never know
ID (Incidence Density) = # of new cases in a given time / total person-time of observation
Which is better?
Incidence: If we were interested in the
natural history of a disorder how it
changes over time
Lead to hypotheses s for future
research
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Prevalence: If you would be more concerned about the number of patients seeking service the
burden of the disorder on society
Duration
How long the disorder lasts
Duration = prevalence / incidence
Prevalence = incidence * duration
Incidence = prevalence / duration
Disorders such as schizophrenia and HIV.AIDS, that have a low incidence but long duration,
have a high prevalence
Foe the common cold, which has a very short duration, the prevalence may be low, but the
incidence high
Measures of mortality
Mortality rate
Reflects how many people die from it in a given time, which is usually one year
MR = # of deaths due to a disorder in a given time / # of people at risk
# of people at risk is the general population
Few problems
1. The number by itself is difficult to interpret, and is useful only when it can be
compared to mortality rates for other disorders
2. Our interest is rarely the population at large, but rather the people who have the
disorder*
Case fatality rate
Solve the second problem* by restricting the denominator to only those who have the
disorder
The proportion of people with disorder who die within a give time, usually one year
CFR = # of deaths due to a disorder in a given time / # of people with the disorder
Another problem: we do not always know how many people there are with a given
disorder
Proportional mortality rate
look at death rates for a specific disorder in relation to deaths from all causes
PMR = # of deaths due to a disorder in a given time / # of deaths from all disorders
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Document Summary

Chapter 10 measures of frequency, effect, and outcomes. Prevalence rate: the proportion of people at risk who have a specific condition at a given time, prevalence rate = # of people with the disorder / # of people at risk in a given interval. Time 2, 8/100: point prevalence vs. period prevalence, lifetime prevalence counts the # of people who have ever had the disorder at any time in their lives. Lifetime prevalence can never decrease as people get older; it can only increase or remain the same. Because once a person has a disorder, he or she should have it for life. Incidence rate: looks at only the new cases. Some people already had the condition at the start of the interval (time 2, a b f i. Others developed it during the time frame (time 2, d g) Incidence rate = # or new cases / # or people at risk in a given interval.

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