EBM Flashcards
what is evidence based medicine?
the conscientious, explicit and judicious use of current best evidence in making decision about the care of individual patients
what does evidence based medicine consider?
- clinical judgement
- relevant scientific evidence
- patients’ values & preferences
what is epidemiology?
‘the study of the occurrence & distribution of health-related events, states, and processes in specified populations, incl. the study of the determinants influencing such processes, and the application of this knowledge to control relevant health problems’
how do you use EBM in medicine?
- diagnosis
- prognosis
- aetiology
- treatment
how is EBM used in diagnoses?
will the result of this test help me improve the accuracy of my diagnosis?
how is EBM used in prognoses?
how long will a patient with this disease survive?
how is EBM used in aetiology?
what are the risk factors for this disease?
how is EBM used in treatment?
is this treatment better than the existing treatment or no treatment at all (placebo)?
what kind of epidemiological study designs are there?
- observational
- interventional
what are examples of observational study designs?
- descriptive
- ecological
- cross-sectional
- case-control
- cohort
what are examples of interventional study types?
- randomised controlled trials (RCT)
- experiment
what is the Bradford Hill criteria for judging whether an association is causal ?
- risk factor PRECEDES disease
- strong & consistent association
- higher levels of risk factor associated w more disease
risk factor is SPECIFIC to the disease - biological mechanism explains the association
- removal or risk factor prevents/reduces disease
how can we rank epidemiological study designs from strong evidence of causality to weak evidence of causality?
STRONG -> WEAK:
RCTs > cohort study > case control study > cross-sectional study
what are cross-sectional studies?
- measure PREVALENCE of disease in a population
- OR examine relationship between diff variables
- ‘clinical iceberg’
what is prevalence?
number with disease at a particular time __________________________________________
total number in a population at that time
what is the clinical iceberg?
tip of iceberg that is seen: known to medical services
unable to see: not seeking advice but aware of illness, diseased but not aware of illness, clinically well
what is a confounding factor?
a confounder is a THIRD variable that provides an alternative explanation for an association between two factors (deciding if its reverse causality or condounder)
what is a case control study?
researchers identify group of individuals with a particular disease/condition (cases), and COMPARE to a group without the disease/condition (controls)
what is the aim of a case control study?
to identify factors/exposures that may be associated w the disease/condition. look at possible reverse causality of factors, and considers confounding factors also
what is a prospective cohort study?
group of individuals who do NOT have a particular disease/condition of interest are followed over time to determine whether they develop it or not
what are prospective cohort studies used for?
to identify risk factors for diseases or conditions and to study the natural history of diseases
what is a RCT?
- gold standard in assessing efficacy of interventions
- designed to minimise biases & provide a rigorous assessment of cause & effect
- investigate effectiveness of new medical treatment/intervention
how does randomisation work?
- details of everyone taking part in the trial are put into a computer
- computer puts each person into a treatment group at random
- computer programme takes into account details such as age & stage of cancer to make sure groups are as similar as possible
what are the types of variables?
- numerical
- categorical
what are numerical variables?
- continuous
- discrete (counts)
what are categorical variables?
- ordered categorial
- unordered categorical
- dichotomous/binary
what are examples of continuous (numerical) variables?
height, weight
what are examples of discrete (numerical) variables?
no. of children in a family
what is an example of ordered categorical variables?
severity of disease - mild, moderate, severe
what is an example of unordered categorical variables?
ethnicity
what is an example of a dichotomous/binary variable?
sex
what type of variable is social class?
ordered categorial
what type of variable is platelet count?
continuous, although theoretically could be discrete
- regarded as continuous bc large no of possible values
what type of variable is post treatment mortality?
binary
what type of variable is educational level?
ordered categorical
what graphs are representative of categorical data?
bar chart, pie chart
what graphs are representative of continuous data?
histograms
what are descriptive statistics for binary variables?
prevalence, risk, incidence rate
how can we calculate risk?
no. of NEW cases of disease in a time period
______________________________________________
no. initially free of disease
what is the incidence rate?
how fast new cases of disease are occurring
how can we calculate incidence rate?
number of new cases of disease
_________________________________________
no initially disease free x TIME INTERVAL
what is the relationship between prevalence and incidence?
under certain conditions,
prevalence = incidence x average disease duration
what are measures of effect?
risk difference
risk ratio
what is risk difference?
ABSOLUTE MEASURE that assesses the difference in risk between the treated/exposed group & the control/non-exposed group
units (e.g. risk per 100, or 1000)
how can you calculate risk difference?
risk in treated/exposed - risk in controls/unexposed
what is the risk ratio?
RELATIVE MEASURE of strength of association between a treatment/exposure and a disease outcome
(no units)
how can we calculate the risk ratio?
risk in treated/exposed / risk in controls/non-exposed
how can we interpret risk ratios?
scale of 0-2
what does it mean if risk ratio is 1?
there is NO difference in risk between groups
what does it mean if risk ratio is less than 1?
treatment/exposure is associated with a REDUCED risk of outcome
e.g. if risk ratio is 0.75 = a 25% reduction in risk of outcome
what if risk ratio is more than 1?
treatment/exposure is associated with an INCREASED risk of the outcome
e.g. risk ratio is 1.75 = 75% increase in risk of outcome
what are descriptive studies?
- describe patterns of disease in terms of time, place, or person
- DON’T consider association or causative (aetiological) hypotheses
what is a major advantage of conducting cross-sectional studies?
they detect cases which may have not presented otherwise
clinical iceberg: would pick up those who are aware of illness but not sought advice, or those diseased but not yet aware of illness
how can we conduct a cross-sectional study?
- define a target population
- define a selected sample
- carry out the study using a study sample
what is a target population?
population to which inferences from study sample are to be made
what is a selected sample?
representative sample of individuals from target population
what is a study sample?
individuals who provide the study data
how can we use statistics to make inferences about the population from the sample?
population -> random sampling -> sample -> statistics -> back to population
what is standard deviation?
measure of the spread of INDIVIDUAL observed values about the mean
what is the 95% reference range?
95% of observed values lie between the mean -1.96 SD and mean +1.96 SD
i.e. the reference range
what is sampling variation?
when the sample means/proportions are different every time when we take repeated random samples from the population
how can we normally distribute sample means/proportions
by making samples large enough
mean of sample means = population mean
how can we estimate a population mean, or a proportion from a single sample?
-> rely on a SINGLE sample, & a SINGLE mean/proportion estimates from that sample
don’t know the variation in the population = use the variation in the sample (STANDARD DEVIATION) as estimation
what is the standard error (SE)?
the standard deviation (SD) of the sampling distribution of a statistic
what does the SE measure?
the prevision of the sample mean as an estimate of the population mean
what is the 95% confidence interval?
when sample means/proportions are normally distributed, 95% will lie between
1.96 x SE and sample statistic + 1.96 x SE
why do we use 1.96 for the 95% confidence interval?
bc 95% of individual sample means/proportions are + or -1.96 standard errors of the mean of the sampling distribution
why is the 95% confidence interval used?
can be 95% confident the interval actually DOES contain the true value of the population statistic
IF study is unbiased
how can we used the 95% confidence interval (C.I.) to assess our sample statistic?
NARROWER the confidence interval = GREATER the prevision of sample statistic
(as estimate of the target population statistic)
what is a disadvantage of 95% C.I.?
doesn’t tell us about ACCURACY
(whether the range of values actually includes population value)
how is accuracy determined?
by how representative the study sample is of the target population
influenced by participants selected into the study
what is falsification?
much easier to find evidence AGAINST a hypothesis than to prove it to be true
- essence of the scientific process
(for a theory to be scientific, MUST be possible to test if it is false)
what does the null hypothesis state about exposure v outcome?
states there is NO ASSOCIATION between an exposure & an outcome
-> can use data to look for evidence AGAINST null hypothesis (falsification)
what are P values?
used to investigate hypotheses
probability of finding the observed (or more extreme) difference if null hypothesis is true
what does the P value tell us?
the strength of evidence AGAINST the null hypothesis; and that the true difference in the population is zero
how does P value relate to null hypothesis?
smaller the P value = stronger evidence AGAINST null hypothesis
(from 0.01 –> 0.0001)
what does it mean if P value is 0.05?
5% probability that the observed difference is due to chance if the null hypothesis is true
(i.e. 1/20 times we would be wrong)
what P value would provide strong value against null hypothesis?
P<0.001
i.e. less than 1 in a 1000 chance of being wrong
what semi-standard structure do research questions often follow?
PICO
what is PICO?
Patient, Intervention, Comparison & Outcome:
‘In [PATIENT] what is the effect of [INTERVENTION] or exposure compared with [COMPARISON] on [OUTCOME]?’
what are measures of effect in RCTs?
risk ratio
risk differences
what does Pearson’s correlation coefficient indicate?
the degree to which two variables have a linear relationship (ranges from -1 to +1)
what do ecological studies examine?
the correlation between average exposure in populations and overall frequency of disease in the populations
what is ecological fallacy?
the assumption that the average characteristics of populations are applicable to individuals within the population
what is linear regression?
describes the linear relationship between one variable and another using the equation of a straight line
what is the equation of the regression line
y = a + bx
outcome = y-axis intercept + (slope of line x exposure)
what does linear regression allow for?
the estimation of the change in value of y (outcome) per unit change in x (exposure)
how can we determine association/causality?
- is it result of chance?
- is it due to bias?
- is it due to confounding?
- is it an example of reverse causality?
- is it causal?
what is the criteria of a confounder?
3rd variable must
- be associated with exposure
- be a risk factor for the disease
- not be on causal pathway between exposure & disease
how can we deal with confounding?
control in the study design (randomisation in RCT)
control in the analysis (stratification)
what is stratification?
within each stratum everybody has the same value of the confounder
what bias can we encounter in RCTS?
confounding
selection bias
loss to follow up bias
performance bias
detection bias
how can we solve confounding in RCTs?
randomisation
how can we solve selection bias in RCTs?
ensure that the allocation sequence is concealed from clinicians/researchers who recruit participants into the trial
how can we solve loss to follow up bias in RCTs?
intention to treat analysis
how can we solve performance bias & detection bias in RCTs?
ensure patients & outcome assessors are unaware of treatment allocation
how can we deal with confounding bias in cohort studies?
adjust for confounders in analysis
how can we deal with selection bias / loss to follow up bias in cohort studies?
minimise losses to follow up during the design & conduct of the study
what is non-differential misclassification (bias)?
misclassification of exposure/outcome occurs randomly
leads to underestimation of true effect of exposure/risk factor on an outcome
how can we deal with non-differential misclassification (bias) in cohort studies?
ensure that exposures & outcomes are measured accurately and consistently between participants
careful design, testing, & standardisation of data collection instruments
what type of bias do we encounter in cohort studies?
confounding
selection bias (loss to follow up)
non-differential misclassification
what type of bias are in case-control studies?
confounding
selection bias
recall bias, interviewer bias
how can we deal with confounding (bias) in case-control studies?
adjust for confounders in analysis
how can we deal with selection bias in case-control studies?
ensure that controls are a representative sample from the at-risk population
how can we deal with recall & interviewer bias in case-control studies?
interviewers must be careful to ask each study participant (cases & controls) in the same way so as not to influence their responses
use STANDARD questionnaires
what is the Bradford Hill criteria?
set of guidelines developed to help determine whether an observed association between an exposure and an outcome is causal
(a framework for evaluating the strength of evidence for causation)
what does the Bradford Hill criteria include?
temporal sequence
strength of the association
consistency of the association
biological gradient (dose-response)
specifity
coherence
reversibility
what is public health?
‘the science & art of preventing disease, prolonging life and promoting health through the organised efforts of society’
what is the focus of public health?
to improve the health of ENTIRE POPULATIONS rather than individual patients
(aims to encompass the whole clinical iceberg)
who is the patient in public health?
the population
who does public health want to target?
ALL ILL HEALTH, INCL.
- asymptomatic/prodromal population
- population who have not yet presented to medical services
- population already being managed by health care services
what are the three domains of public health?
health improvement
health protection
healthcare public health
what is health improvement?
- wider factors that affect health & wellbeing
- healthy lifestyles & choices
- inequalities
what is healthcare public health?
- disease prevention
- service improvement
- evidence based practise
- equity of provision
what is health protection?
- infectious diseases
- emergency response
- environmental hazards
what is the bell-curve shift in populations?
shifting the whole population into a lower risk category benefits more individuals than shifting high risk individuals into a lower risk category
what are examples of tools for improving population health?
- health improvement; schools & workplaces
- screening programmes; detect disease at an early (treatable) stage
- immunisation; prevent transmission of infection diseases
- legislation; prohibit actions or behaviours (e.g. smoking in enclosed public places)
- promote healthy actions/behaviours; e.g. extend cycle lanes to encourage active transport
- development of new clinical services for treating disease
what are the types of disease prevention?
- primary
- secondary
- tertiary
what is primary disease prevention?
aims to prevent ONSET of disease
may alter environmental factor/change behaviour
what is secondary prevention?
aims to HALT progression of disease
focuses on early detection or diagnosis followed by prompt & effective treatment
may be aimed at asymptomatic people
what is tertiary prevention?
focus on treatment and rehabilitation of people w established disease
aims to minimise complications & disability
what are the advantages of cross-sectional studies?
relatively quick & inexpensive to conduct - useful tool for generating hypotheses for further research
what are the disadvantages of cross-sectional studies?
cannot establish causality or directionality of relationships between variables
may be subject to selection bias if sample is not representative of population of interest
may not account for changes in prevalence of disease/condition over time
