Trials.

Report
Understanding real
research 4.
Randomised controlled trials.
What can studies do?
Describe the situation: Descriptive.
Explain the situation:
Analytical.
Compare approaches: Experimental.
Study designs
Descriptive
Cross-sectional, longitudinal.
Analytic
Case-control studies.
Cohort studies.
Quasi-experimental
Natural experiments, policy interventions.
Experimental
Randomised controlled trial.
Study designs
Type of Study
Descriptive
Analytical
Experimental
Case study
Yes
No
No
Case series
Yes
No
No
Cross-sectional
Yes
Yes
No
Case-control
Yes
Yes
No
Cohort
Yes
Yes
No
Natural experiment
Yes
Yes
Quasi
Randomised control trial
Yes
Yes
Yes
Prevalence
Cross-sectional
Cause/
Aetiology
Cross-sectional;
Case-control;
Cohort.
Prognosis
Cohort.
Harm
Case-control;
Cohort.
Effectiveness
Randomised controlled
trial.
Randomised controlled trials
A clinical trial in which:
•at least two treatments, programmes,
interventions are compared.
•one of these is a control group.
•allocation uses a random, unbiased method.
Randomised controlled trials
New treatment
Group 1
Outcome
Group 2
Outcome
Population
Control treatment
From: Critical Appraisal Skills Programme (CASP), Oxford.
Explanatory trials
Measure efficacy:
the benefit a treatment produces under ideal
conditions.
e.g. Phase III drug trials.
Pragmatic trials
Measure effectiveness:
the benefit a treatment produces in routine clinical
practice.
Aim to inform choices between treatments.
Patients should be analysed in the group to which they
were initially randomised, i.e. intention to treat analysis.
Intention to treat analysis
All patients allocated to one arm of a RCT
are analysed in that arm, whether or not
they completed the prescribed
treatment/regimen.
Two by two table
Outcome event
Total
Yes
No
Experimental
group
a
b
a+b
Control group
c
d
c+d
a+c
b+d
a + b +c + d
Total
Appraising RCTs
Methodological approach:
Was assignment to the different groups
randomised?
Was the randomisation process/list concealed?
Was everyone who entered the trial accounted for
at the end?
Were subjects and assessors “blind” to treatment
allocation when assessing outcomes?
Were groups similar at start of trial and treated
similarly throughout the study?
Appraising RCTs
Statistical reporting:
Were subjects analysed in the group to which
they were randomised: intention to treat
analysis.
Type of data – influences statistical analysis.
Reporting of risk: RRR vs ARR.
Risks and Odds.
Risks and odds
When talking about the chance of something
happening, e.g. death, hip fracture, we can
talk about:
• risk and relative risk
or
• odds and odds ratio.
Risks and odds
Risks.
A proportion.
Numerator / Denominator.
Odds.
A ratio.
Numerator / (Denominator - Numerator).
Two by two table
Outcome event
Total
Yes
No
Experimental
group
a
b
a+b
Control group
c
d
c+d
a+c
b+d
a + b +c + d
Total
Risk
Risk is: a proportion.
Risk of event in expt. group =
Risk of event in control group =
a = EER.
a+b
c = CER.
c+d
Relative risk
Relative risk (RR) is: a ratio of proportions.
RR = EER
CER.
A measure of the chance of the event occurring
in the experimental group relative to it occurring
in the control group.
Relative risk - 2
RR <1 if group represented in the numerator is at
lower “risk” of the event.
Want this if the event is a bad outcome e.g. death.
RR >1 if group represented in numerator is at greater
“risk” of the event.
Want this if the event is a good outcome e.g. smoking
cessation.
Relative risk reduction
The amount by which the risk of the event is
reduced by the intervention.
The difference in the risk of the event
between the control and experimental groups,
relative to the control group.
RRR = (CER - EER)/CER.
Use this term if the event is bad e.g. death.
Relative risk reduction - 2
An alternative way of calculating the relative
risk reduction is to use the relative risk:
RRR = (1 - RR).
Use this term if the event is bad e.g. death.
Absolute risk reduction
The absolute difference between the risk of
the event in the control and experimental
groups.
ARR = CER - EER.
ARR can be used to calculate the number
needed to treat (NNT).
Use this term if the event is bad e.g. death.
Relative benefit increase
The amount by which the risk of the event is
increased by the intervention.
The difference in the risk of the event between
the control and experimental groups, relative to
the control group.
RBI = (CER - EER)/CER.
Use this term if the event is good e.g. smoking
cessation.
Relative benefit increase - 2
An alternative way of calculating the relative
benefit increase is to use the relative risk:
RBI = (1 - RR).
Use this term if the event is good e.g. smoking
cessation.
Absolute benefit increase
The absolute difference between the risk of the
event in the control and experimental groups.
ABI = CER - EER.
ABI can be used to calculate the number needed to
treat (NNT).
Use this term if the event is good e.g. smoking cessation.
Number needed to treat
The number of patients who needed to be
treated to prevent the occurrence of one adverse
event (e.g. complication, death) or promote the
occurrence of one beneficial event (e.g.
cessation of smoking).
NNT = 1/ARR.
Odds.
Odds is: a ratio.
Odds of event in expt. group =
Odds of event in control group =
a
b.
c
d.
Odds ratio.
Odds ration (OR) is: a ratio of ratios.
OR = ad
bc.
Confidence interval
The range of values within which the “true” value in the
population is found.
95% CI: can be 95% confident the population value
lies within those limits.
Is an estimate of the “true” value.
Confidence interval - 2
95% CI = Sample estimate +/- 1.96 x SE
The bigger the sample - the smaller the sample error
(SE).
Bigger samples  smaller CIs.
more precise estimate of the “true”
population value.

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