CAT Flashcards
What are case-control studies?
Observational and retrospective
They examine an association between an outcome and exposure to a risk factor
The studies recruit participants based on the presence or absence of an outcome. Data of exposure to a risk factor is then recorded retrospectively for each of the participants and compared between cases and controls
It asks ‘ what happened?’
What do case-control studies measure?
Odds ratios
What are cohort studies?
Observational and prospective/retrospective
They compared 2 groups, one with and one without exposures and examine them over a period of time to establish links between exposure and a health related outcome
It asks “what will happen”?
What do cohort studies measure?
Relative risk
What are cross-sectional studies?
Observational
A sample of individuals from a population is selected and data is collected on both the exposure of interest and outcome of interest. This allows estimations of prevalence and risk factors
Asks “what is happening?”
What are advantages and disadvantages of case-control studies?
Advantages:
- good for investigating rare outcomes as these are identified at the start
- quicker than cohort studies and easier - don’t require long follow ups
- relatively inexpensive
- good for long latent period
- can study multiple exposures associated with a disease
Disadvantages:
- recall bias
- not suitable when exposure to risk is rare
- subject to selection bias
- difficult to establish causality as exposure data is collected retrospectively i.e. did exposure precede disease
Whats an example of a case-control study in medical research?
a study investigating the association between smoking and lung cancer. In this study, cases would be individuals diagnosed with lung cancer, and controls would be individuals without lung cancer. Researchers would then collect information on the smoking history of both cases and controls, including the number of cigarettes smoked per day, the duration of smoking, and the age at which individuals started smoking.
The researchers would then compare the smoking history of cases and controls to determine whether there is an association between smoking and lung cancer. If the study finds that individuals who smoke have a higher risk of developing lung cancer compared to those who do not smoke, this would provide evidence for the harmful effects of smoking on lung health.
What are the advantages and disadvantages of cohort studies?
Advanatges:
- can establish temporal relationships between exposure and outcome (best information about causation)
- can study multiple outcomes associated with a particular exposure
- can study rare exposures
- can provide incidence rates
Disadvantages:
- long follow up - expensive ans time consuming
- attrition bias
- selection bias
- they rely on accurate measurement of exposure and may be subject to misclassification bias if expsoure is measured inaccurately
- not suitable for rare outcomes
- bad for long latency periods
Whats an example of a cohort study in medical research?
Framingham Heart Study, which began in 1948 and has followed several generations of participants in the town of Framingham, Massachusetts, to investigate the risk factors for cardiovascular disease. This study has been instrumental in identifying risk factors for heart disease, such as high blood pressure and cholesterol, and has contributed to the development of interventions and treatments for these conditions.
What are advantages and disadvantages of cross-sectional studies?
Advantages:
- quick
- inexpensive
- can study multiple outcomes
Disadvantages:
- cannot establish causality (can’t determine whether exposure or outcome came first)
- subject to selection bias as sample may not be representative of entire population
- cannot study rare outcomes are sample size unlikely to be large enough
What are the key differences between observational studies and experimental studies?
Observational studies are designed to observe and measure outcomes in a population without intervention. The goal of observational studies is to identify associations between exposures or risk factors and outcomes. These studies can be further divided into cross-sectional, case-control, and cohort studies.
Experimental studies, on the other hand, are designed to test hypotheses by manipulating one or more variables and measuring the effect on the outcome. Experimental studies can be further divided into randomized controlled trials (RCTs) and non-randomized controlled trials
What are the advantages and disadvantages of observational studies?
Advantages:
- cost effective
- real world settings which increases generalisability
- can study rare outcomes which would not be feasible in an experimental setting
Disadvanatges
- limited control so difficulty in drawing conclusions about causality
- prone to confounding
- cannot establish causality
What are the advantages and disadvantages of experimental studies?
Advantages:
- control - which increases the ability to establish causality
- eliminates confounding factors by methods such as blinding - increases internal validity
- can be replicated more easily which increases the reliability of findings
Disadvantages:
- expensive due to costs of intervention
- often conducted in artificial setting which may limit the generalisability
- ethical considerations e.g. use of placebo group
What are the types of experimental studies?
RCTs
Quasi-experimental studies
Single-subject experimental designs
What are RCTs?
These studies are considered the gold standard in experimental research. Participants are randomly assigned to either an experimental group or a control group, with the experimental group receiving the intervention or treatment being tested, while the control group receives a placebo or standard treatment. RCTs are designed to minimize bias and confounding variables and to ensure that any observed effects are due to the intervention being tested.
What are Quasi-experimental studies?
These studies are similar to RCTs but lack the random assignment of participants to groups. Participants are assigned to either an experimental or control group based on non-random methods, such as geographical location or medical history. While these studies are less rigorous than RCTs, they can still provide valuable information about the effectiveness of interventions.
What are single-subject experimental designs?
These studies involve one or a few participants who receive multiple measurements of a treatment or intervention over time. This design is particularly useful for studying rare or complex conditions and allows for the evaluation of individual responses to a treatment.
What are advantages and disadvantages of RCTs?
Advanatges:
- high internal validity
- ability to establish causation
- reproducible
- generalisable
Disadvanatges:
- expensive
- time consuming
- ethical concerns about withholding treatment from control group
- limited external validity
- attrition and non-compliance
What is bias?
Systematic (as opposed to random) deviation of the results of a study from the ‘true’ results, which is caused by the way the study is designed or conducted.
What is selection bias?
Non-random assigning individuals to groups, leading to differences in group’s qualities that may influence the outcomes
I.e. the subjects are not representative of the population
What is recall bias?
Difference in accuracy of recollection of study participants - may be due to time or influenced by motive (i.e. knowledge of presence of disorder alters recall by subjects)
Which study type is most prone to recall bias?
Case-control studyes
What is publication bias?
Failure to publish or include certain studies because they have negative results
Occurs when issues other than the quality of the study are allowed to influence the decision to publish
Which study type is publication bias most important in?
Systematic reviews and meta-analyses (where studies showing negative results may be excluded)
What is the Hawthorne effect?
The group changing their behaviour due to knowledge they are being studied
What is procedure bias?
Subjects in different groups recieving different care/not treated the same, other than just the intervention
E.g. subjects in treatment group get more attention which stimulates greater compliance
What is information bias?
when information used in a study is either measured or recorded inaccurately.
What is observer bias?
Aka pygmalion effect or expectation bias
When observers subcuncuously measure or report data in a way that favours the expected study outcome
Only a problem in non-blinded trials
What is confirmation bias?
the tendency to interpret new evidence as confirmation of one’s existing beliefs or theories.
What is attrition bias?
a type of selection bias due to systematic differences between study groups in the number and the way participants are lost from a study.
Which type of study is most prone to selection bias?
Cohort studies
What is work-up bias?
Aka verification bias
a type of measurement bias in which the results of a diagnostic test affect whether the gold standard procedure is used to verify the test result.
Seen in studies trying to validate a new diagnostic test
In clinical practice, verification bias is more likely to occur when a preliminary diagnostic test is negative. Because many gold standard tests can be invasive, expensive, and carry a higher risk (e.g. angiography, biopsy, surgery), patients and physicians may be more reluctant to undergo further work-up if a preliminary test is negative.
What is randomisation?
Randomization is the process of assigning participants to treatment and control groups, assuming that each participant has an equal chance of being assigned to any group
What are the types of randomisation?
Simple randomisation
Stratified randomisation
Block randomisation
Covariate adaptive randomisation
What is simple randomisation?
This is the most basic type of randomization, where each individual or item in a population has an equal chance of being selected.
E.g. computer random number generator
What is stratified randomisation?
This method is used when a population can be divided into subgroups or strata based on certain characteristics. The individuals are then randomly assigned to the treatment or control group within each stratum, ensuring that each group is representative of the strata.
What is blocked randomisation?
Individuals are groups based on their variable characteristics and then within these groups there will be random allocation to treatment or control group within each block
This method is used to ensure that the treatment and control groups have an equal number of individuals with certain characteristics or variables.
What is cluster randomisation?
This method is used when individuals cannot be randomized individually but rather in groups or clusters, such as schools or neighborhoods. The clusters are then randomly assigned to the treatment or control group.
What is covariate adaptive randomisation?
a type of adaptive randomization that uses pre-specified participant characteristics, or covariates, to determine the allocation of participants to treatment groups in a clinical trial. This method is used to improve the balance between treatment groups with respect to known prognostic factors that may affect the outcome of the study.
What is clinical equipoise?
This means that there is genuine uncertainty in the expert medical community over whether a treatment will be beneficial
This is lost when there is prior reason to believe 1 intervention is superior/inferior to the other
Whats the problem with inadequate randomisation?
If randomization is inadequate, then there may be systematic differences between the groups that are not related to the treatment being studied.
This can lead to biased estimates of treatment effects and reduce the internal validity of the study. Inadequate randomization can also increase the risk of type I and type II errors, where false positive or false negative results are obtained.
What is blinding?
a method used in research studies to prevent bias by ensuring that participants, researchers, and outcome assessors do not know which treatment group a participant has been assigned to.
Blinding can be single-blind, where only the participants or the assessors are unaware of the treatment group, or double-blind, where both the participants and the assessors are unaware of the treatment group.
Why is blinding important?
Prevents observer bias, confirmation bias and prevents placebo effect
What is intention to treat analysis?
An assessment of the people taking part in a trial, based on the group they were initially (and randomly) allocated to. This is regardless of whether or not they dropped out, fully adhered to the treatment or switched to an alternative treatment. Intention-to-treat analysis analyses are often used to assess clinical effectiveness because they mirror actual practice, when not everyone adheres to the treatment, and the treatment people have may be changed according to how their condition responds to it.
What is power of a study?
the probability of rejecting the null hypothesis when it is in fact false
What is usually an acceptable power value?
80-90%
What is the P-value?
a statistical measure that indicates whether or not an effect is statistically significant.
p value <0.05 it is considered that there probably is a real difference between treatments.
p value is 0.001 or less the result is seen as highly significant.
What are confidence intervals?
A way of expressing how certain we are about the findings from a study, using statistics. It gives a range of results that is likely to include the ‘true’ value for the population
The CI is usually stated as ‘95% CI’, which means that the range of values has a 95 in a 100 chance of including the ‘true’ value.
What does the width of CI indicate?
. A wide confidence interval (CI) indicates a lack of certainty about the true effect of the test or treatment - often because a small group of patients has been studied. A narrow CI indicates a more precise estimate (for example, if a large number of patients have been studied).
What is an underpowered trial?
a clinical trial that has an insufficient sample size or statistical power to detect a true effect of the intervention being studied.
A study with low statistical power has a high risk of yielding a false negative result, meaning that it fails to detect a true effect even when one exists. (Type 2 statistical error)
What is subversion bias?
The deliberate manipulation of pt recruitment by clinicians in order to influence trial results e.g. shining a light through an envelope to determine the patient’s allocation group
What is standard deviation?
A value that shows how much variation there is from the mean
What is validity?
Whether a test or study actually measures what it aims to measure
what is the difference between internal and external validity?
Internal validity shows whether a study or test is appropriate for the question, for example, whether a study of exercise among gym members measures the amount of exercise people do at the gym, not simply whether people join.
External validity is the degree to which the results of a study hold true in non-study situations, for example, in routine NHS practice. It may also be referred to as the generalisability of study results to non-study populations. For example, the external validity of a study that took place in Spain may be questioned if the results were to be applied to people in Australia.
What is a type 1 error?
Aka false positive
occurs when the null hypothesis is rejected even though it is true. In other words, stating there is an effect when none exists
What is a type 2 error?
Aka false negative
occurs when the null hypothesis is not rejected even though it is false. In other words, stating there is not an effect when one exists
What is the null hypothesis?
the hypothesis that there is no significant difference between specified populations, any observed difference being due to sampling or experimental error.
It is usually denoted as H0
What is the alternative hypothesis?
Hypothesis that there is some assocation between disease and risk factor
It is usually denoted as H1 and is the hypothesis that researchers want to support by rejecting the null hypothesis
What methods are used to limit confounding in a cohort study?
Restriction
Matching
Stratification
Multiple variable regression
Propensity score matching
Sensitivity analysis
What is restriction?
Limiting participants of study who have possible confounders - means less data!
What is matching and stratification?
You make comparison groups i.e. groups with and without the confounder, adjust for confounding and then recombine the data
What is multivariable regression?
Coefficients are established for the confounder groups
This allows for better adjustment
What is a meta-analysis?
A statistical technique often used in systematic reviews to ‘pool’ the results from several studies of the same test, treatment or other intervention to estimate the overall summary effect of the treatment.
This provides a more precise estimate of treatment effect than a single study would
(In mark scheme it says you need to say pool and precise for both marks!)
What are the pros and cons of a meta-analysis?
Pros:
Increased statical power
Increases generalisability by including studies with diverse populations, settings and study designs
Highly transparent - provides a systematic and objective approach to summarising evidence
Higher precision due to higher number of pt
Cons:
Limited by heterogeneity
Publication bias
Dependant on the quality of studies included and low quality studies can bias the summary effect size
Dependant on data availability
what is a systematic review?
Its an overview that answers a specific clinical question and contains a thorough, unbiased search of the relevant literature, explicit criteria for assessing studies and a structured presentation of the results
It may or may not use statistical techniques, such as meta-analysis.
Whats are the pros and cons of systematic review?
Pros:
- Conducts a thorough search for evidence which can eminiate publication bias, language bias and selection bias
- Increases total sample size which improves power = improves certainty and precision
- replicable and transparent which increases the credibility and trustworthiness
- identifies gaps in literature
Cons:
- time consuming and resource intensive
- limited by data availability
- limited by study design
- risk of bias in selection and interpretation of studies
What is the correlation coefficient?
a number between +1 and −1
a statistical measure of the strength of a linear relationship between two variables
Whats the most appropriate study type to answer a question on diagnosis?
Cross-sectional analytic study
Whats the most appropriate study type to answer a question on aetiology?
Cohort study>Case-control study
Whats the most appropriate study type to answer a question on prognosis?
Cohort study > case control
Whats the most appropriate study type to answer a question on treatment?
RCT or systematic review of RCT ? Cohort > case control
What is treatment fidelity? Why is it important?
the degree to which an intervention or treatment is implemented as intended or designed. It ensures its conducted consistently and reliably
Treatment fidelity is important because it allows researchers to evaluate the effectiveness of an intervention and determine whether any observed effects are due to the treatment itself, rather than differences in how the treatment was delivered
What are the Bradford-Hill criteria?
Aset of nine criteria to provide epidemiologic evidence of a causal relationship between a presumed cause and an observed effect
- strength - the large the association, the more likely it is causal
- consistency - consistent findings in different studies strengthens likelihood of causality
- specificity - the more specific the association is the bigger the probability of causal effect
- temporality - the effect can to occur after the cause
- biology gradient - greater exposure should generally lead to greater incidence of effect
- plausibility - a plausible mechanism between cause and effect is helpful
- coherence - coherence between epidemiological and lab findings increases the likelihood
- experiment
- analogy - use of analogies or similarities between observed association
What is confounding?
A third factor which is associated with both the exposure and outcome that can potentially distort the association between outcome and exposure
What happens if follow-up was not complete enough in a cohort study?
Allows for selection bias and may reduce the power of the study
It can introduce selection bias because participants who are lost to follow-up may differ systematically from those who remain in the study, and this can affect the validity of the results.
What is propensity score matching?
a statistical technique that can be used to balance the distribution of confounding variables between the exposed and unexposed groups.
In an observational study, participants are not randomly assigned to treatment or control groups, which can lead to confounding factors that may affect the outcome of interest. Propensity score matching involves estimating the probability of receiving the treatment based on observed covariates or variables that may be related to both treatment assignment and the outcome. The propensity score is then used to match participants in the treatment group with participants in the control group who have similar scores, based on a specified level of similarity. This creates a comparison group that is more similar to the treatment group in terms of the distribution of potential confounding variables, which can reduce bias and improve the validity of the study results.
