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