Flashcards in The Use of Data Deck (67):
Definition of Disease
Symptoms, signs - diagnosis: a bio-medical perspective
Definition of Illness
Ideas, concerns, expectations - experience: patients perspective
What factors affect the uptake of care (4)
Concept of Lay referral - i.e. granny knows best
Sources of info - peers, family, internet, TV, newspaper
Medical - new symptoms, visible symptoms, increasing severity, duration
Non-medical - crisis, peer pressure (wife sent me), patient beliefs, expectations, social class, economic, psychological, environmental, cultural, ethnic, age, gender, media
Possible isues from the patient about care (2)
Believes to be healthy because is physically fit.
Proud not to be using tablets
Possible issues from the GP about care (3)
Wish to perform a couple more tests – a Holter Monitor and an Echocardiogram – why might you do these tests?
Worried about the consequences for long term health.
What are you concerned about, and what sources of information might you have used to educate yourself about that?
What are the 3 main epidemiological aims of being able to provide information to patients (3)
Description - to describe the amount and distribution of disease in human populations.
Explanation - to elucidate the natural history and identify aetiological factors for disease usually by combining epidemiological data with data from other disciplines such as biochemistry, occupational health and genetics.
Disease control - to provide the basis on which preventive measures, public health practices and therapeutic strategies can be developed, implemented, monitored and evaluated for the purposes of disease control.
What does epidemiology compare? (1 + 3)
Groups (study populations) in order to detect differences pointing to –
- Aetiological clues (what causes the problem)
- The scope for prevention
- The identification of high risk or priority groups in society.
Which fundamental measures does epidemiology use to study disease and risk?
Talk in terms of ratio: Numerator / Denominator -
- Event / Population at Risk
- i.e. Deaths from IHD in men aged 55-64 in Grampian in 1990 / All men aged 55-64 in Grampian in 1990
What does the risk part in the ratios of studies of diseases mean
That everyone in the denominator has the potential to move into the numerator and in turn that everyone in the numerator has come from the denominator
What is incidence
The number of new cases of a disease in a population in a specified period of time
What is prevalence
The number of people within a population with a specific disease at a single point in time or in a defined period of time
What may have a high incidence but a low prevalence
Minor illnesses - i.e. cold
What may have a low incidence but a high prevalence
Chronic illnesses - i.e. diabetes
What is relative risk
The measure of the strength of an association between a suspected risk factor and the disease under study
What is the equation of relative risk =
incidence of disease in an exposed group / incidence of disease in an unexposed group
(see word document for example)
10 sources of epidemiological data
Expenditure data from NHS
General practice morbidity
Hospital activity statistics
Reproductive health statistics
Social security statistics
Health and household surveys
Drug misuse databases
What is health literacy
About people having the knowledge, skills, understanding and confidence to use health information, to be active partners in their care, and to navigate health and social care systems.
Health literacy is being increasingly recognised as a significant health concern around the world
What are the different types of studies (1,2(3), 3)
What is a descriptive study
Attempt to describe the amount and distribution of a disease in a given population.
- Follow the time, place, person framework
- May look at disease alone or also look into factors (exposures) thought to be linked to the aetiology
When are descriptive epidemiological studies useful (5)
1. Identifying emerging public health problems by monitoring and surveilling disease patterns
2. Signalling effects worth of further investigation
3. Assessing the effectiveness of measures of prevention and control (eg, screening programmes).
4. Assessing needs for health services and service planning.
5. Generating hypotheses about disease aetiology and risk factors
What 3 things does a descriptive study not provide (3)
1. Definitive conclusions about disease causation
2. Evidence about the causes of disease
3. Do not test hypotheses.
What is a benefit of descriptive studies (3)
3. Give a valuable initial overview of a problem
What do cross-sectional studies look at and what do they conclude
Observations made at a single point in time - disease frequency, survey, prevalence study
Conclusions indicate relationship between diseases and other variables of interest in a defined population.
What is a pro and con to cross-sectional studies
Provide quick results
Is usually impossible to infer causation
What does a case-control study compare
Two groups of people are compared:
1. Group of individuals with the disease of interest are identified (cases)
2. Group of individuals who do not have the disease (controls)
How is data gathered in a case-control study (3 steps)
1. Data is gathered on each individual to determine whether or not they have been exposed to the suspected aetiological factor(s)
2. The average exposure in the two groups, cases and controls is compared
3. This identifies significant differences, give clues to factors which elevate (or reduce) risk of the disease under investigation.
How are the results of a case-control study expressed (3)
1. 'odds ratios'
2. 'relative risks' - also used for cohort & randomised trials
3. Confidence intervals / 'p values' may be presented to guide whether the result could be a chance finding
Steps of a cohort study (2)
1. Baseline data on exposure is collected from a group of people who do not have the disease under study
2. The group is then followed through time until a sufficient number have developed the disease to allow analysis.
3. The original group is separated into subgroups according to original exposure status.
4. These subgroups are compared to determine the incidence of disease according to exposure.
What do cohort studies allow for (2)
1. The calculation of cumulative incidence
2. For differences in follow up time
What is a trial
Experiments used to test ideas about aetiology or to evaluate interventions.
What is the definitive method of assessing any new treatment in medicine
Randomised control trial
How does a randomised control trial work (4)
1. Two groups at risk of developing a disease are assembled: a study (intervention) group and a control group.
2. An alteration is made to the intervention group (eg, a suspected causative factor is removed or neutralised), whilst no alteration is made to the control group.
3. Data on subsequent outcomes (eg, disease incidence) are collected in the same way from both groups
4. The relative risk is calculated.
What is the aim of randomised control trial
To determine whether modification of the factor (removing, reducing or increasing exposure) alters the incidence of the disease.
How is a randomised control trial of new treatment used
The underlying design is the same:
- the intervention group receive the new therapy
- the control group receive the current standard therapy (or a placebo)
- the treatment outcomes (eg, reduction in symptoms) are compared in the two groups.
What 6 factors need to be considered when interpreting the results of a study
2. Standardised Mortality Ratio
3. Quality of Data
4. Case Definition
5. Coding and Classification
What is standardisation and give an example
1. A set of techniques used to remove or adjust for the effects of differences when comparing two or more populations, in age, social class etc.
Age-sex standardised rate: represents what the unstandardised (crude) rate would have been in the study population, if that population had the same proportion of males and females, and of people in different age groups, as the standard population
What is standardised mortality ratio (2) and give an example
1. A standardised death rate converted into a ratio for easy comparison.
2. The figure for a standard reference population (eg, Scotland) is taken to be 100 and the standardised death rates for the comparison (study) populations (eg, Grampian) are expressed as a proportion of 100.
- A figure below one hundred means fewer than expected deaths, and above 100 means more.
i.e. an SMR of 120 means that 20% more deaths occurred than expected in the study population, allowing for differences in the age and sex structure of the standard and study populations and an SMR of 83 means 17% fewer deaths occurred
What is quality of data
Ensuring that the data are trustworthy
What is case definition
The purpose of case definition is to decide whether an individual has the condition of interest or not
Important as not all investigators mean the same thing when they use medical terms.
What is coding and classification, what does it assist in
1. When data is collected routinely, eg, death certificates, it is normal to convert disease information to a set of codes
2. Assists in data storage and analysis
3. Rules are drawn up to dictate how clinical information is converted to a code
(Related to the case definition)
What happens to the incidence of disease if the rules of how to code clinical information is changed
If these rules change, it sometimes appears that a disease has become more or less common, when in fact it has just been coded under a new heading.
What is ascertainment and how may it effect incidence
Is the data complete - are any subjects missing?
If researchers in one country look harder for cases of a given disease than researchers in any other, it might not be surprising that they come up with higher incidence rates.
What is bias
Any trend in the collection, analysis, interpretation, publication or review of data that can lead to conclusions that are systematically different from the truth.
What are the four different types of bias
1. Selection Bias
2. Information Bias
3. Follow up Bias
4. Systematic Error
What is selection bias
Occurs when the study sample is not truly representative of the whole study population about which conclusions are to be drawn.
What is information bias
Arises from systematic errors in measuring exposure or disease.
What is follow-up bias
Arises when one group of subjects is followed up more assiduously than another to measure disease incidence or other relevant outcome.
What is systematic error bias
A form of measurement bias where there is a tendency for measurements to always fall on one side of the true value.
Example of selection bias
In a randomised controlled trial of a new drug, subjects should be allocated to the intervention (study) group and control group using a random method.
If certain types of people (eg, older, more ill) were deliberately allocated to one of these groups then the results of the trial would reflect these differences, not just the effect of the drug.
Example of information bias
In a case control study, a researcher who was aware of whether the patient being interviewed was a 'case' or a 'control' might encourage cases more than controls to think hard about past exposures to the factors of interest.
Any differences in exposure would then reflect the enthusiasm of the researcher as well as any true difference in exposure between the two groups.
Example of follow-up bias
In cohort studies, subjects sometimes move address or fail to reply to questionnaires sent out by the researchers.
If greater attempts are made to trace these missing subjects from the group with greater initial exposure to a factor of interest than from the group with less exposure, the resulting relative risk would be based on a (relative) underestimate of the incidence in the less exposed group compared with the more exposed group.
Example of systematic error bias
It may be because the instrument (eg, a blood pressure machine) is calibrated wrongly, or because of the way a person uses an instrument.
This problem may occur with interviews, questionnaires etc, as well as with medical instruments.
What is a confounding factor
One which is associated independently with both the disease and with the exposure under investigation and so distorts the relationship between the exposure and disease.
In some cases, the confounding factor may be the true causal factor, and not the exposure that is under consideration.
What are common confounders (3)
What are 5 ways to deal with confounding
1. Randomisation in trials
2. Restriction of eligibility criteria to only certain kinds of study subjects .
3. Subjects in different groups can be matched for likely confounding factors.
4. Results can be stratified according to confounding factors.
5. Results can be adjusted (using multivariate analysis techniques) to take account of suspected confounding factors.
Why is their a criteria for causality
It is difficult to prove causation between an exposure and disease and the criteria help to assess the available evidence
What are the 9 criteria for causality
1. Strength of association
5. Biological gradient
6. Biological plausibility
What is strength of association
As measured by relative risk or odds ratio.
What is consistency
Repeated observation of an association in different populations under different circumstances.
What is specificity
A single exposure leading to a single disease.
What is temporality
The exposure comes before the disease.
What is biological gradient
Dose-response relationship. As the exposure increases so does the risk of disease.
What is biological plausibility
The association agrees with what is known about the biology of the disease.
What is coherence
The association does not conflict with what is known about the biology of the disease.
What is analogy
Another exposure-disease relationship exists which can act as a model for the one under investigation.
i.e. it is known that certain drugs can cross the placenta and cause birth defects
- it might be possible for viruses to do the same.
What is experiment
A suitably controlled experiment to prove the association as causal - very uncommon in human populations.