Measures of disease frequency - Revision history

Gauche at 15:50, 9 May 2011

2011-05-09T15:50:59Z

Gauche at 15:42, 4 May 2011

2011-05-04T15:42:36Z

Gauche at 15:40, 4 May 2011

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Gauche at 15:33, 4 May 2011

2011-05-04T15:33:25Z

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2011-05-04T15:32:55Z

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2011-05-04T15:04:51Z

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2011-05-04T14:15:45Z

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2011-05-04T14:03:43Z

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2011-05-04T13:43:53Z

Gauche at 13:35, 4 May 2011

2011-05-04T13:35:28Z

← Older revision		Revision as of 15:50, 9 May 2011
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	The hazard function describes the pattern of the 'incidence rate' over time (rather than summing an 'average' estimate over the duration of the study, as with the incidence rate itself). Although the hazard function may be described graphically, it is most commonly used in analytic studies, when hazard functions are compared in the form of a hazard ratio (a [[Measures of strength of association\|measure of strength of association]]).		The hazard function describes the pattern of the 'incidence rate' over time (rather than summing an 'average' estimate over the duration of the study, as with the incidence rate itself). Although the hazard function may be described graphically, it is most commonly used in analytic studies, when hazard functions are compared in the form of a hazard ratio (a [[Measures of strength of association\|measure of strength of association]]).

−	[[Category:Veterinary Epidemiology - Statistical Methods\|A]]	+	[[Category:Veterinary Epidemiology - Statistical Methods\|B]]

@@ Line 16: / Line 16: @@
 ==Survival time analysis==
-Survival analysis (also known as the '''time-to-event''' analysis) includes similar concepts to both the incidence risk and the incidence rate, but adopts a different approach to investigation and interpretation. Survival time analysis can be recognised as comprising two different measurements: the '''survivor function''' (and '''survival time''') and the '''hazard function'''. As for incidence measurement, these measures are based upon following an initially disease-free population of animals over time, and so require studies such as [[Study design#Cohort studies|cohort studies]].
+Survival analysis (also known as the '''time-to-event''' analysis) includes similar concepts to both the incidence risk and the incidence rate, but adopts a different approach to investigation and interpretation. Survival time analysis can be recognised as comprising two different measurements: the '''survivor function''' (and '''survival time''') and the '''hazard function'''. Of these, the survivor function is most commonly used as a method of data description, with the hazard function being used for comparison of disease experience between groups in the case of analytic studies. As for incidence measurement, these measures are based upon following an initially disease-free population of animals over time, and so require studies such as [[Study design#Cohort studies|cohort studies]].
 ===Survivor function===
@@ Line 22: / Line 22: @@
 ===Hazard function===
-The hazard function describes the pattern of the 'incidence rate' over time (rather than summing an 'average' estimate over the duration of the study, as with the incidence rate itself). It may be described graphically, but is most commonly used in analytic studies, when hazard functions are compared in the form of a hazard ratio (a [[Measures of strength of association|measure of strength of association]]).
+The hazard function describes the pattern of the 'incidence rate' over time (rather than summing an 'average' estimate over the duration of the study, as with the incidence rate itself). Although the hazard function may be described graphically, it is most commonly used in analytic studies, when hazard functions are compared in the form of a hazard ratio (a [[Measures of strength of association|measure of strength of association]]).
 [[Category:Veterinary Epidemiology - Statistical Methods|A]]

@@ Line 19: / Line 19: @@
 ===Survivor function===
-The survival function (also known as the '''cumulative survival probability''') describes the proportion of initially disease-free individuals who have not experienced the outcome of interest over time, and so is similar conceptually to the incidence risk. The survival time may be visualised using a Kaplan-Meier curve, or may be summarised using the ''survival time''. Survival time is measured in units of time and describes the time until animals experience disease. Commonly, the '''median survival time''' is presented, which is the time at which 50% of the population have experienced the event (this is the most appropriate measure of the 'average' survival time, as the mean would not be able to account for individuals who have not experienced the outcome by the end of the study). However, if 50% of the individuals have not experienced disease by the end of the study, the median survival time cannot be calculated. Another option is to describe the proportion of individuals who have not experienced disease at a set point in time, which is the same concept as the [[#Incidence risk|incidence risk]]
+The survival function (also known as the '''cumulative survival probability''') describes the proportion of initially disease-free individuals who have not experienced the outcome of interest over time, and so is similar conceptually to the incidence risk. The survival time may be visualised graphically (using a Kaplan-Meier curve, for example) or may be summarised using the ''survival time''. Survival time is measured in units of time and describes the time until animals experience disease. Commonly, the '''median survival time''' is presented, which is the time at which 50% of the population have experienced the event (this is the most appropriate measure of the 'average' survival time, as the mean would not be able to account for individuals who have not experienced the outcome by the end of the study). However, if 50% of the individuals have not experienced disease by the end of the study, the median survival time cannot be calculated. Another option is to describe the proportion of individuals who have experienced/not experienced disease at a set point in time, which is the same concept as the [[#Incidence risk|incidence risk]].
 ===Hazard function===
+The hazard function describes the pattern of the 'incidence rate' over time (rather than summing an 'average' estimate over the duration of the study, as with the incidence rate itself). It may be described graphically, but is most commonly used in analytic studies, when hazard functions are compared in the form of a hazard ratio (a [[Measures of strength of association|measure of strength of association]]).
 [[Category:Veterinary Epidemiology - Statistical Methods|A]]

@@ Line 19: / Line 19: @@
 ===Survivor function===
-The survival function (also known as the '''cumulative survival probability''') describes the proportion of initially disease-free individuals who have not experienced the outcome of interest over time, and so is similar conceptually to the incidence risk. The survival time may be visualised using a Kaplan-Meier curve, or may be summarised using the ''survival time''. Survival time is measured in units of time and describes the time until animals experience disease. Commonly, the '''median survival time''' is presented, which is the time at which 50% of the population have experienced the event (this is the most appropriate measure of the 'average' survival time, as the mean would not be able to account for individuals who have not experienced the outcome by the end of the study). However, if 50% of the individuals have not experienced disease by the end of the study, the median survival time cannot be calculated. Another option is to describe the proportion of individuals who have not experienced disease at a set point in time, which is the same concept as the [[incidence risk|Incidence risk]]
+The survival function (also known as the '''cumulative survival probability''') describes the proportion of initially disease-free individuals who have not experienced the outcome of interest over time, and so is similar conceptually to the incidence risk. The survival time may be visualised using a Kaplan-Meier curve, or may be summarised using the ''survival time''. Survival time is measured in units of time and describes the time until animals experience disease. Commonly, the '''median survival time''' is presented, which is the time at which 50% of the population have experienced the event (this is the most appropriate measure of the 'average' survival time, as the mean would not be able to account for individuals who have not experienced the outcome by the end of the study). However, if 50% of the individuals have not experienced disease by the end of the study, the median survival time cannot be calculated. Another option is to describe the proportion of individuals who have not experienced disease at a set point in time, which is the same concept as the [[#Incidence risk|incidence risk]]
 ===Hazard function===

@@ Line 1: / Line 1: @@
 A very common aim of epidemiological investigation is to estimate the frequency of disease in a population. This is of particular importance in the case of [[Surveillance and monitoring of disease|surveillance and disease monitoring systems]], and is commonly the central aim of many [[Descriptive epidemiological studies|descriptive studies]]. There are two main measures of disease frequency used by epidemiologists - the '''prevalence''' and the '''incidence''' of disease, which each measure different aspects of disease. The '''survival time''', which is closely associated with the incidence, is another measure commonly used. Counts of disease are not commonly used in epidemiological studies, although they can be useful when deciding upon resource requirements when implementing disease control strategies.
 ==Prevalence==
 The prevalence (also known as the ''prevalence risk''') of a disease is the proportion of animals in the population of interest which are 'diseased' at any specific point in time, and is calculated by dividing the number of cases of disease by the total size of the population. This may be a single point (in which case, the measure is the '''point prevalence'''), or may be over a period (in which case the '''period prevalence''' is given). However, it is important to realise that this measure will therefore include both old and new cases of disease, which can be a problem when attempting to investigate risk factors for a disease of long duration (as many of the diseased animals are likely to have had the disease for some time). The prevalence can be viewed at an individual animal level as the 'probability that an animal selected at random from the population will be diseased'. The prevalence of disease is commonly obtained from [[Study design#Surveys|surveys]] or [[Study design#Cross sectional studies|cross sectional studies]], although in some cases, [[Study design#Cohort studies|cohort studies]] may be appropriate (dependent on how animals were selected).
@@ Line 13: / Line 15: @@
 The incidence rate is a measure of the frequency of disease in relation to both the number of nondiseased animals and the total amount of time for which these animals were followed, meaning that the time period does not need to be provided in order for it to be interpretable. It can also be viewed as the 'instantaneous rate of disease' at any point over the period of investigation (and therefore assumes that this rate is constant over the period). It is calculated by dividing the number of cases of disease over the period of study with the total number of '''animal-time units''' which were contributed over this period. These animal-time units are estimated by adding together the total amount of time which each animal was present in the study for (i.e. until animals experienced disease, left the study for other reasons, or the study concluded) for each animal in the study. Examples of animal-time units may be cow-days, or dog-weeks, or horse-years, and so on. As each individual animal contributes its own number of animal-time units, animals which enter the study population during the study period can be included, as can animals which experience more than one episode of disease (in which case, it must be ensured that animals which have experienced disease and are susceptible again contribute animal-time units to the denominator). Unlike the prevalence and the incidence risk, the incidence rate has no interpretation at the individal animal level, only at the population level.
-==Survival time==
+==Survival time analysis==
-Survival analysis (also known as the '''time-to-event''' analysis) includes similar concepts to both the incidence risk and the incidence rate, but adopts a different approach to investigation and interpretation. Three approaches to survival time analysis can be recognised: measurement of the '''survival time''', measurement of the '''survivor function''' and measurement of the '''hazard function'''. As for incidence measurements, these measures are based upon following an initially disease-free population of animals.
+Survival analysis (also known as the '''time-to-event''' analysis) includes similar concepts to both the incidence risk and the incidence rate, but adopts a different approach to investigation and interpretation. Survival time analysis can be recognised as comprising two different measurements: the '''survivor function''' (and '''survival time''') and the '''hazard function'''. As for incidence measurement, these measures are based upon following an initially disease-free population of animals over time, and so require studies such as [[Study design#Cohort studies|cohort studies]].
-==Survival time==
+===Survivor function===
-The survival time is measured in units of time and describes the time until animals experience disease. Commonly, the median survival time will be presented, which is the time at which 50% of the population
+The survival function (also known as the '''cumulative survival probability''') describes the proportion of initially disease-free individuals who have not experienced the outcome of interest over time, and so is similar conceptually to the incidence risk. The survival time may be visualised using a Kaplan-Meier curve, or may be summarised using the ''survival time''. Survival time is measured in units of time and describes the time until animals experience disease. Commonly, the '''median survival time''' is presented, which is the time at which 50% of the population have experienced the event (this is the most appropriate measure of the 'average' survival time, as the mean would not be able to account for individuals who have not experienced the outcome by the end of the study). However, if 50% of the individuals have not experienced disease by the end of the study, the median survival time cannot be calculated. Another option is to describe the proportion of individuals who have not experienced disease at a set point in time, which is the same concept as the [[incidence risk|Incidence risk]]
-==Survivor function==
+===Hazard function===
 [[Category:Veterinary Epidemiology - Statistical Methods|A]]

@@ Line 1: / Line 1: @@
-A very common aim of epidemiological investigation is to estimate the levels of disease in a population. This is of particular importance in the case of [[Surveillance and monitoring of disease|surveillance and disease monitoring systems]], and is commonly the central aim of many [[Descriptive epidemiological studies|descriptive studies]]. There are two main measures of disease frequency used by epidemiologists - the '''prevalence''' and the '''incidence''' of disease, which each measure different aspects of disease. The '''time to event''', which is closely related to the incidence rate, is another measure commonly used. Counts of disease are not commonly used in epidemiological studies, although they can be useful when deciding upon resource requirements when implementing disease control strategies.
+A very common aim of epidemiological investigation is to estimate the frequency of disease in a population. This is of particular importance in the case of [[Surveillance and monitoring of disease|surveillance and disease monitoring systems]], and is commonly the central aim of many [[Descriptive epidemiological studies|descriptive studies]]. There are two main measures of disease frequency used by epidemiologists - the '''prevalence''' and the '''incidence''' of disease, which each measure different aspects of disease. The '''survival time''', which is closely associated with the incidence, is another measure commonly used. Counts of disease are not commonly used in epidemiological studies, although they can be useful when deciding upon resource requirements when implementing disease control strategies.
 ==Prevalence==
@@ Line 5: / Line 5: @@
 ==Incidence==
-The incidence of disease describes the frequency of new cases of disease amongst previously nondiseased animals, and there are two main methods of measuring it - known as the '''incidence risk''' and the '''incidence rate'''.
+The incidence of disease describes the frequency of new cases of disease amongst previously nondiseased animals, and there are two main methods of measuring it - known as the '''incidence risk''' and the '''incidence rate'''. The incidence can only be measured from studies which follow animals up over time, such as [[Study design#Cohort studies|cohort studies]] and [[Study design#Experimental studies|randomised controlled trials]].
 ===Incidence risk===
@@ Line 11: / Line 11: @@
 ===Incidence rate===
-The incidence rate is a measure of the frequency of disease in relation to both the number of nondiseased animals and the total amount of time for which these animals were followed, meaning that the time period does not need to be provided in order for it to be interpretable. It can also be viewed as the 'instantaneous rate of disease' at any point over the period of investigation. It is calculated by dividing the number of cases of disease over the period of study with the total number of '''animal-time units''' which were contributed over this period. These animal-time units are estimated by adding together the total amount of time which each animal was present in the study for (i.e. until animals experienced disease, left the study for other reasons, or the study concluded) for each animal in the study. Examples of animal-time units may be cow-days, or dog-weeks, or horse-years, and so on. As each individual animal contributes its own number of animal-time units, animals which enter the study population during the study period can be included, as can animals which experience more than one episode of disease (in which case, it must be ensured that animals which have experienced disease and are susceptible again contribute animal-time units to the denominator). Unlike the prevalence and the incidence risk, the incidence rate has no interpretation at the individal animal level, only at the population level.
+The incidence rate is a measure of the frequency of disease in relation to both the number of nondiseased animals and the total amount of time for which these animals were followed, meaning that the time period does not need to be provided in order for it to be interpretable. It can also be viewed as the 'instantaneous rate of disease' at any point over the period of investigation (and therefore assumes that this rate is constant over the period). It is calculated by dividing the number of cases of disease over the period of study with the total number of '''animal-time units''' which were contributed over this period. These animal-time units are estimated by adding together the total amount of time which each animal was present in the study for (i.e. until animals experienced disease, left the study for other reasons, or the study concluded) for each animal in the study. Examples of animal-time units may be cow-days, or dog-weeks, or horse-years, and so on. As each individual animal contributes its own number of animal-time units, animals which enter the study population during the study period can be included, as can animals which experience more than one episode of disease (in which case, it must be ensured that animals which have experienced disease and are susceptible again contribute animal-time units to the denominator). Unlike the prevalence and the incidence risk, the incidence rate has no interpretation at the individal animal level, only at the population level.
-==Time to event==
+==Survival time==
 ==Counts==

@@ Line 2: / Line 2: @@
 ==Prevalence==
-The prevalence of a disease is the proportion of animals in the population of interest which are 'diseased' at any specific point in time. This may be a single point (in which case, the measure is the '''point prevalence'''), or may be over a period (in which case the '''period prevalence''' is given). However, it is important to realise that this measure will therefore include both old and new cases of disease, which can be a problem when attempting to investigate risk factors for a disease of long duration (as many of the diseased animals are likely to have had the disease for some time). The prevalence can be viewed at an individual animal level as the 'probability that an animal selected at random from the population will be diseased'. The prevalence of disease is commonly obtained from [[Study design#Descriptive studies#Surveys|surveys]] or cross sectional studies, although in some cases, it can be estimated from a cohort study.
+The prevalence of a disease is the proportion of animals in the population of interest which are 'diseased' at any specific point in time. This may be a single point (in which case, the measure is the '''point prevalence'''), or may be over a period (in which case the '''period prevalence''' is given). However, it is important to realise that this measure will therefore include both old and new cases of disease, which can be a problem when attempting to investigate risk factors for a disease of long duration (as many of the diseased animals are likely to have had the disease for some time). The prevalence can be viewed at an individual animal level as the 'probability that an animal selected at random from the population will be diseased'. The prevalence of disease is commonly obtained from [[Study design#Surveys|surveys]] or cross sectional studies, although in some cases, it can be estimated from a cohort study.
 ==Incidence==