Phase 3a Public Health Minerva Revision Resource ppt Flashcards
What is case control study
retrospective studies that take people with a disease (case) and match them to people without the disease for age/sex/habitat/class etc (control) and study previous exposure to the agent in question.
Advantages of case control?
Goor for rare outcomes (eg cancer)
Quicker than cohort or intervention studies *as outcome has already happened)
Can investigate mutliptle exposures
Disadvantages of case control
Difficulties finding controls to match with cases
Prone to selection and information bias
What is cohort study
start with a population without the disease in question and study them over time to see if they are exposed to the agent in question and if they develop the disease in question or not
Advantages of cohort study
Can follow-up with a rare exposure (eg natural disaster)
Good for common and multiple outcomes
Less risk of selection and recall bias
Disadvantages of cohort
Takes a long time
Loss to follow up (people drop out)
Need large sample size
What is cross sectional study
Divides population into those without the disease and those with the disease and collect data on them once at a defined time to find associations at that point in time.
Advantages of cross sectional
Quick and cheap
Provides data on prevalence at single point in time
Large sample size
Good for surveillance and public health planning
Disadvantages of cross sectional
Risk of reverse causality (dont know whether outcome or exposure came first)
Cannot measure incidence
Risk recall bias and non-response
What is RCT
patients are randomised into groups, one group is given an intervention, the other is given a control and the outcome is measured
Advantages of RCT
Low risk of bias and confounding
Can infer causality (gold standard)
Disadvantages of RCT
Time consuming
Expensive
Specific inclusion/exclusion criteria may mean study population different from typical patients (eg excluding elderly people)
Give 5 reasons as to why a study may find an association between an exposure and outcome
Chance Bias Confounding Reverse causality True casual association
List 3 types of bias
Selection
Information
Publication
What is selection bias
A systematic error in:
- selection of study participants
- allocation of participants to different study groups
3 Examples of selection bias
- Non-response (are those who don’t respond to postal surveys more likely to be ill, elderly, from lower socioeconomic groups?)
- Loss to follow up (are those receiving an intervention more likely to drop out of the study because they feel better/worse/suffer side effects/move into residential care?)
- Are those in the intervention group (or the cases) different in some way from the controls other than the exposure in question?
List 4 types of information bias and give example of each
- Measurement (e.g. different equipment used to measure the outcome in the different groups)
- Observer (e.g. the researcher knows which participants are cases and which are controls and subconsciously reports/measures the exposure or outcome differently depending on which group they are in)
- Recall (e.g. events that happened in the past are not remembered and reported accurately)
- Reporting (e.g. respondents report inaccurate information because they are embarrassed or feel judged)
2 facts about publication bias
- Drug trials with unfavourable results less likely to be published - particularly so if pharma company funded the trial
- There is less incentive to conduct research on non-pharmacological treatments; drug companies are willing to fund studies to prove efficacy of their products but not interested in funding non-pharmacological studies
What is confounding
A situation in which the estimate between an exposure and an outcome is distorted because of the association of the exposure with another factor (confounder) that is also independently associated with the outcome.
what is reverse causality
This refers to the situation when an association between an exposure and an outcome could be due to the outcome causing the exposure rather than the exposure causing the outcome
Name of criteria used to determine the strength of association between particular factor and outcome
Bradford-Hill
9 aspects of Bradford-Hill criteria
It is difficult to prove that an exposure causes an outcome, but the following conditions increase the likelihood of this being the case:
- Strength. Stronger association between the exposure and the outcome. E.g. Heavy alcohol consumption is associated with a ten times greater odds of laryngeal cancer.
- Consistency. Same result observed from various studies and in different geographical settings. E.g. The association between cigarette smoking and cardiovascular disease has been observed in many cohort and case-control studies over 30 years in different populations.
- Dose-response. Increased risk of outcome with increased exposure. E.g. Heavy smoking is associated with an increased risk of lung cancer compared to moderate smoking (which is in turn associated with greater risk than light smoking)
- Temporality. Exposure occurs prior to outcome. E.g. A cohort initially exposed to nuclear radiation is subsequently more likely to develop cancer
Not easy in case-control and cross-sectional studies because exposure and outcome measured simultaneously - Plausibility. Reasonable biological mechanism
Depends on existing knowledge - Reversibility. Intervention to reduce/remove exposure eliminates/reduces outcome. E.g. Randomised intervention trials of vitamin D supplementation in the elderly found it improves muscle strength and function, supporting evidence that vitamin D deficiency can cause muscle weakness
- Coherence. Logical consistency with other information. E.g. The increase in smoking habits and incidence of lung cancer over time is consistent with laboratory evidence that cigarette smoke is a risk factor for cancer in animals
- Analogy. Similarity with other established cause-effect relationships. E.g. Previous research concluding that thalidomide in pregnancy causes birth defects supports new, weaker evidence of a similar drug causing similar effects
- Specificity. Relationship specific to outcome of interest. E.g. After introducing bike helmets is there a reduction in head injuries greater than any background reduction in cycling injuries more generally?
What is the aim of screening
to identify apparently well individuals who have (or are at risk of developing) a particular disease so that you can have a real impact on the outcome
NHS criteria for a screening test (based on Wilson and Junger)
Condition
- Important health problem
- Epidemiology and natural history of condition should be adequately understood and there should be a detectable risk factor, disease marker, latent period and early symptomatic stage
- All the cost-effective primary prevention interventions should have been implemented as far as practicable
- If carriers of a mutation are identified as result of screening, the natural history of people with this status should be understood, including the psychological implications
Screening programme
- Should be ongoing not just performed on one-off basis
- Costs should be economically balanced in relation to healthcare spending as a whole
The test
- Simple, safe, precise and validated screening test
- Distribution of test values in target population should be known and suitable cut off level defined and agreed
- Should be acceptable to population
- Agreed policy on further diagnostic investigation of individuals with a positive test result and on the choices available to those individuals
- If the test is for mutations the criteria used to select the subset of mutations to be covered by screening, if all possible mutations are not being tested, should be clearly set out (eg cystic fibrosis screening only includes most common mutations)
The treatment
- Should be an effective treatment or intervention for patients identified through early detection, with evidence of early treatment leading to better outcomes than late treatment
- Should be agreed evidence based policies covering which individuals should be offered treatment and the appropriate treatment to be offered
- Clinical management of the condition and patient outcomes should be optimised in all health care providers prior to participation in a screening programme
Pneumonic: I Understand SCREEN Important, understand natural history, sensitive test, common problem, risk outweigh benefits, early/latent phase, expense low, non-invasive treatment
Disadvantages of screening
- Exposure of well individuals to distressing or harmful diagnostic tests (eg colonoscopy following positive faecal occult blood tests)
- Detection and treatment of sub-clinical disease that would never have caused any problems (eg non-aggressive prostate ca in elderly men)
- Preventive interventions that may cause harm to individual/population (E.g. the potential for increased antibiotic resistance if all mothers were screened for group B streptococcus in pregnancy (this is not currently carried out in the UK, in contrast to the USA)
Define sensitivity, give equation
Proportion of those with the disease who are correctly identified by the screening test (if this is too low, you will miss too many cases)
Sens = TP/(TP + FN) or number of true positives/number of people with disease
Define specificity, give equation
Proportion of people without the disease who are correctly excluded by the screening test (if this is too low you will have many people who undergo unnecessary diagnostic interventions because they don’t have the disease)
Spec = TN/(TN + FP) or number of true negatives/number of people without disease
Define PPV, give equation
Proportion of people with a positive test result who actually have the disease (this is higher if the prevalence is higher
PPV = TP/(TP + FP) or number of true positives/number of true and false positives
- Predictive values dependent on underlying prevalence – sens & spec are not
High prevalence -> false positives fall -> PPV increase and NPV decreases
Define NPV, give equation
Proportion of people with a negative test result who do not have the disease (this is lower if the prevalence is higher)
NPV = TN/(TN + FN) or number of true negatives/number of true negatives and false negatives
- Predictive values dependent on underlying prevalence – sens & spec are not
High prevalence -> false positives fall -> PPV increase and NPV decreases
Define lead time bias
When screening identifies an outcome earlier than it would otherwise have been identified, this results in an apparent increase in survival time, even if screening has no effect on outcome
This raises the question as to whether it is beneficial to identify diseases early in cases where this will result in patients spending longer in the knowledge that they have the disease but does not improve our ability to treat it.
Define length time bias
Type of bias resulting from differences in the length of time taken for a condition to progress to severe effects, that may affect the apparent efficacy of a screening method
E.g. Consider two groups of women with cervical cancer. One group had their cancer detected when they had their cervical smears (screening test). They had low grade cancers that progressed at a slow rate so they had not become symptomatic between the screening tests. The second group were diagnosed after they became symptomatic (e.g. post-coital bleeding). They had high grade cancers that were so aggressive they progressed to a symptomatic stage within the three years between smear tests. It appears that screening results in cancers with a better prognosis but in fact this is due to the fact that screening tends to detect less aggressive cancers.
Name 3 approaches for health needs assessment
- Epidemiological
- Comparative
- Comparative
List the factors involved in epidemiological approach
Disease incidence & prevalence
Morbidity & mortality
Life expectancy
Services available (location, cost, utilisation, effectiveness etc)
Sources of data: disease registry, hospital admissions, GP databases, mortality data, primary data collection (e.g. postal/patient survey)
Advantages of epidemiological approach
Uses existing data
Provides data on disease; Incidence/mortality/morbidity
Can evaluate services by trends over time
Disadvantages of epidemiological approach
Quality of data variable
Data collected may not be data required
Does not consider felt needs or opinions/experiences of people affected
List factors involved in corporate approach
- Ask local population what their health needs are
- Use focus groups, interviews, public meetings
- Wide variety of stakeholders eg teachers, healthcare professional, social workers, charity workers, local businesses, council workers
Advantages of corporate approach
- Based on felt and expressed needs of population in question
- Recognise the detailed knowledge and experience of those working with the population
- Takes into account a wide range of views
Disadvantages of corporate approach
- Difficult to distinguish need from demand
- Groups may have vested interests
- May be influenced by political agendas
Lists factors in comparative approach
- Compare health or healthcare provision of one population to another
- Spatial (eg different town) or social (eg age, social class)
- Can compare health, service provision/utilisation, health outcomes
- Means of evaluating variation in performance/costs of services
Advantages of comparative approach
- Quick and cheap if data available
- Indicates whether health or services provision is better/worse than comparable areas (gives a measure of relative performance)
Disadvantages of comparative approach
- May be difficult to find comparable population
- Data may not be available/high quality
- May not yield what the most appropriate level (eg of provision or utilisation) should be
Define public health
Science and art of preventing disease, prolonging life and promoting health through organised efforts of society
Define and give examples of primary prevention
Primary prevention aims to prevent disease or injury before it ever occurs. This is done by preventing exposures to hazards that cause disease or injury, altering unhealthy or unsafe behaviours that can lead to disease or injury, and increasing resistance to disease or injury should exposure occur.
Examples include:
legislation and enforcement to ban or control the use of hazardous products (e.g. asbestos) or to mandate safe and healthy practices (e.g. use of seatbelts and bike helmets)
education about healthy and safe habits (e.g. eating well, exercising regularly, not smoking)
immunization against infectious diseases.
Define and give examples of secondary prevention
Secondary prevention aims to reduce the impact of a disease or injury that has already occurred. The focus of secondary prevention is early disease detection, making it possible to prevent the worsening of the disease and the emergence of symptoms, or to minimize complications and limit disabilities before the disease becomes severe
Examples include:
regular exams and screening tests to detect disease in its earliest stages (e.g. mammograms to detect breast cancer)
daily, low-dose aspirins and/or diet and exercise programs to prevent further heart attacks or strokes
suitably modified work so injured or ill workers can return safely to their jobs.
Define and give example of tertiary prevention
The goal of tertiary prevention is to reduce the negative impact of an already-established disease by restoring function and reducing disease-related complications.2 Tertiary prevention also aims to improve the quality of life for people with disease.
Examples:
- cardiac or stroke rehabilitation programs, chronic disease management programs (e.g. for diabetes, arthritis, depression, etc.)
- support groups that allow members to share strategies for living well
- vocational rehabilitation programs to retrain workers for new jobs when they have recovered as much as possible.
List 2 types of screening approaches
Population approach
High risk approach
Define population approach to screening (including its effect on risk factor distribution)
preventative measure delivered on a population wide basis and seeks to shift the risk factor distribution curve, e.g. dietary salt reduction through legislation, working with the food industry and advice to the general public should shift the blood pressure distribution curve to the left.
Define high risk approach to screening
identify individuals above a chosen cut-off and treat them, e.g. screening for people with high blood pressure and treating them.
What is the prevention paradox (described by Rose)
A preventive measure which brings much benefit to the population often offers little to each participating individual
Define screening
process which sorts out apparently well people who probably have a disease (or precursors or susceptibility to a disease) from those who probably do not.
Not intended to be diagnostic.
What are the types of screening
- Population-based programmes
- Opportunistic
- Screening for communicable disease
- Pre-employment and occupational medicals
- Commercially provided screening
Define health needs assessment
A systematic approach for reviewing the health issues affecting a population which leads to agreed priorities and resource allocation that will improve health and decrease inequalities
