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17 Cards in this Set
- Front
- Back
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What is prevalence? |
What proportion of the population have the condition at time t - point prevalence |
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Why do we use prevalence? |
-Assessing health status of a population -Comparing impact of condition among different populations -Planning health services |
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Equation for prevalence |
#of affected individuals total population at time t
P=I*D |
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Downfalls of prevalence |
Disease duration Can't imply causation Survivor affect
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What is incidence? |
The occurance of new cases of a condition in a population during a specific period
-Cummulative Incidence -Incidence Rate |
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What is cumulative incidence? |
The proportion of an outcome-free population that develops the outcome of interest in a specified time period. |
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What is cumulative incidence used for? |
Risk - shows average probability of a person in the population developing the condition during the period studied. Provides clues as to what could be causing a condition as shows the development of a condition. Unaffected by disease duration. |
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Equation for cumulative incidence |
#of people who developed the condition in time t population at risk of developing the condition at the begining of time t |
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Issues with cumulative incidence |
Assumes a 'closed' population Highly dependent on time period - greater time= greater cumulative incidence |
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What is incidence rate? |
The rate at which new cases of the outcome of interest occur in a population. |
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Person-time at risk |
The sum of the lengths of time spent at risk of becoming a case for everyone in the study population. |
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How do people stop being at risk? |
When they become a new case They are lost to follow up (death, moved away) The period of follow up ends |
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Why is incidence rate used? |
Accounts for people being at risk for different lengths of time. Indicates average rate of new cases during a time period. Accounting for different lengths of time at risk Identifiing determinants of a condition |
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Reporting incidence rate |
Eg The incidence rate of cancer in villagers was 67 per 100 person-years |
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Issues with Incidence Rate |
More complex study - expensive Person time isn't always available |
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The relationship between incidence and prevalence |
Prevalence - Incidence * Average disease duration (in a stable population with rare diseases) |
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Age standardisation |
Used when comparing two groups with different age structures and the condition studied varies with age eg arthritis Makes the age structures of both populations the same. |
