Epidemiological Studies
Content
Types of Epidemiological studies
Epidemiology - Terms
• Epidemiology comes from Greek
– Epidemos (“on people”)
• Related to the words
– Epidemic – Demographics
• Epidemiology is the study of diseases in populations of humans or animals
– How – When – Where, they occur
Epidemiologists
• Epidemiologists attempt to determine what factors are associated with disease
– RISK FACTORS
• Also what factors protect people against disease
– PROTECTIVE FACTORS
• The first epidemiologists looked at possible causes of contagious diseases
– Smallpox – Typhoid – Polio
Modern Epidemiology
• Epidemiology has been expanded to study non-transmissible disease
– Cancer – Poisonings due to environmental agents
• Molecular epidemiology focused on the biochemical and genetic changes associated with specific diseases • Specific biochemical “footprints” indicate different types of exposure of cellular damage.
– Eg. Cyclobutane pyrimidine dimer DNA damage and UV exposure
Epidemiology Can:Cannot
• Epidemiological studies cannot prove the causation of any disease. • They cannot say that a specific risk factor causes the disease being studied. • Epidemiological studies can only show that the risk factor is associated or correlated with a higher incidence of the disease in the exposed population.
Examples
• The link between smoking and lung cancer. • Based upon comparisons of rates of lung cancer in smokers and non-smokers • Rates are much higher in populations which smoke. • This does not prove smoking causes lung cancer
– It identifies a RISK FACTOR
Example
• To prove smoking causes lung cancer it was necessary to expose animals to tobacco smoke and smoke extracts. • Needs highly controlled conditions with a defined control group. • Only difference between controls and exposed group is the exposure to smoke etc. • Such studies provided a CAUSAL link between smoking and lung cancer.
Types of Epidemiological Study
• Descriptive Epidemiology • Distribution of cancer cases within a population according to certain characteristics.
– – – – Geographic factors Occupation Age Sex Residence etc
• Risk Factor Epidemiology • Combination of environmental factors associated with the disease
Descriptive Epidemiology
• INCIDENCE
–A measure of frequency of the cancer in a given population
• INCIDENCE RATE
Number of people with cancer in a specific time period Total population at that time Incidence is therefore a PROBABILITY that tries to estimate risk that a person will develop cancer during a given time. Often expressed per 100,000 of the population
Incidence Rates – Breast Cancer
Descriptive Epidemiology
• PREVALENCE: The number of people in the population with cancer at a specific time.
– A proportion and not a rate
• Number of people with cancer • Total population • Annual prevalence is the proportion within a single year • Lifetime prevalence: during a complete lifetime
Descriptive Epidemiology
• Prevalence of cancer with a good prognosis will be higher than incidence (lots of people living with disease). • Low prevalence with poor prognosis (people die). • Cancer generally has a high incidence because it’s a long-term illness
Prevalence = Incidence x duration
Mortality Statistics
• The number of people who die from cancer. • A useful tool in descriptive epidemiology • Expressed as the number of deaths per 100,000 of the population. • The only completely reliable source of information • UK in 2002 there were 155,180 cancer deaths.
Analytical Epidemiology
• Descriptive epidemiology observes trends. Occurrence, prevalence and incidences but cannot identify risks and potential causative factors. • Analytical epidemiology is based upon comparisons of groups of people with different characteristics. • Two methods
– Case controlled studies – Cohort studies
Case-controlled studies
• Investigation into groups of people with cancer and groups of people without cancer • Determine differences in
– – – – – – – – Exposure Occupation Behaviour Lifestyle Geography Location Socio-economic factors Genetic susceptibility etc
Case-controlled Studies
• They are retrospective and data is obtained by various methods (interviews, questionnaires etc) • They estimate RISK or RELATIVE RISK.
Exposed A B Cancer No Cancer C D
Non-exposed
• Relative risk = AxD BxC
Study Bias
• Many different types of bias can affect case-controlled studies. • Difficult to get well matched study groups. • Difficult to control for all patient
– Behaviour – Exposure – Honesty etc.
Cohort Studies
• Cohort studies are prospective studies. • A cohort of healthy people is selected and studied for many years. • The characteristics of those that get cancer in this group are then closely studied. • These characteristics can the be identified and compared to those in the group that do not get cancer
Cohort Studies
• Cohort studies may be much more reliable than case-controlled studies. • They are however
– Time consuming – Complicated – Very expensive
• Most of the bias is eliminated because there is no need for a control group
Cohort Studies
• Fixed cohort studies: people are chosen due to a specific event occurring at a specific time
– Cohort does not take on new members
• Open dynamic cohorts are common in cancer epidemiology • Cohort allows for in and out migration. • New members can be added due to attritional loss.
Other Issues
• Confounding factors
– Unrecognised variable: Smoking often associated with cirrhosis but does not cause it. – Caused by excessive smoking (associated with heavy drinking).
• Molecular epidemiology
– Tumour marker – Mutational gene profile – Toxins
Epidemiology - Terms
• Epidemiology comes from Greek
– Epidemos (“on people”)
• Related to the words
– Epidemic – Demographics
• Epidemiology is the study of diseases in populations of humans or animals
– How – When – Where, they occur
Epidemiologists
• Epidemiologists attempt to determine what factors are associated with disease
– RISK FACTORS
• Also what factors protect people against disease
– PROTECTIVE FACTORS
• The first epidemiologists looked at possible causes of contagious diseases
– Smallpox – Typhoid – Polio
Modern Epidemiology
• Epidemiology has been expanded to study non-transmissible disease
– Cancer – Poisonings due to environmental agents
• Molecular epidemiology focused on the biochemical and genetic changes associated with specific diseases • Specific biochemical “footprints” indicate different types of exposure of cellular damage.
– Eg. Cyclobutane pyrimidine dimer DNA damage and UV exposure
Epidemiology Can:Cannot
• Epidemiological studies cannot prove the causation of any disease. • They cannot say that a specific risk factor causes the disease being studied. • Epidemiological studies can only show that the risk factor is associated or correlated with a higher incidence of the disease in the exposed population.
Examples
• The link between smoking and lung cancer. • Based upon comparisons of rates of lung cancer in smokers and non-smokers • Rates are much higher in populations which smoke. • This does not prove smoking causes lung cancer
– It identifies a RISK FACTOR
Example
• To prove smoking causes lung cancer it was necessary to expose animals to tobacco smoke and smoke extracts. • Needs highly controlled conditions with a defined control group. • Only difference between controls and exposed group is the exposure to smoke etc. • Such studies provided a CAUSAL link between smoking and lung cancer.
Types of Epidemiological Study
• Descriptive Epidemiology • Distribution of cancer cases within a population according to certain characteristics.
– – – – Geographic factors Occupation Age Sex Residence etc
• Risk Factor Epidemiology • Combination of environmental factors associated with the disease
Descriptive Epidemiology
• INCIDENCE
–A measure of frequency of the cancer in a given population
• INCIDENCE RATE
Number of people with cancer in a specific time period Total population at that time Incidence is therefore a PROBABILITY that tries to estimate risk that a person will develop cancer during a given time. Often expressed per 100,000 of the population
Incidence Rates – Breast Cancer
Descriptive Epidemiology
• PREVALENCE: The number of people in the population with cancer at a specific time.
– A proportion and not a rate
• Number of people with cancer • Total population • Annual prevalence is the proportion within a single year • Lifetime prevalence: during a complete lifetime
Descriptive Epidemiology
• Prevalence of cancer with a good prognosis will be higher than incidence (lots of people living with disease). • Low prevalence with poor prognosis (people die). • Cancer generally has a high incidence because it’s a long-term illness
Prevalence = Incidence x duration
Mortality Statistics
• The number of people who die from cancer. • A useful tool in descriptive epidemiology • Expressed as the number of deaths per 100,000 of the population. • The only completely reliable source of information • UK in 2002 there were 155,180 cancer deaths.
Analytical Epidemiology
• Descriptive epidemiology observes trends. Occurrence, prevalence and incidences but cannot identify risks and potential causative factors. • Analytical epidemiology is based upon comparisons of groups of people with different characteristics. • Two methods
– Case controlled studies – Cohort studies
Case-controlled studies
• Investigation into groups of people with cancer and groups of people without cancer • Determine differences in
– – – – – – – – Exposure Occupation Behaviour Lifestyle Geography Location Socio-economic factors Genetic susceptibility etc
Case-controlled Studies
• They are retrospective and data is obtained by various methods (interviews, questionnaires etc) • They estimate RISK or RELATIVE RISK.
Exposed A B Cancer No Cancer C D
Non-exposed
• Relative risk = AxD BxC
Study Bias
• Many different types of bias can affect case-controlled studies. • Difficult to get well matched study groups. • Difficult to control for all patient
– Behaviour – Exposure – Honesty etc.
Cohort Studies
• Cohort studies are prospective studies. • A cohort of healthy people is selected and studied for many years. • The characteristics of those that get cancer in this group are then closely studied. • These characteristics can the be identified and compared to those in the group that do not get cancer
Cohort Studies
• Cohort studies may be much more reliable than case-controlled studies. • They are however
– Time consuming – Complicated – Very expensive
• Most of the bias is eliminated because there is no need for a control group
Cohort Studies
• Fixed cohort studies: people are chosen due to a specific event occurring at a specific time
– Cohort does not take on new members
• Open dynamic cohorts are common in cancer epidemiology • Cohort allows for in and out migration. • New members can be added due to attritional loss.
Other Issues
• Confounding factors
– Unrecognised variable: Smoking often associated with cirrhosis but does not cause it. – Caused by excessive smoking (associated with heavy drinking).
• Molecular epidemiology
– Tumour marker – Mutational gene profile – Toxins
