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Flashcards in Statistics and Critical Appraisal Deck (58)
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what is a proportion?

number of events occuring/number in group


what is risk/prevalence (P)?

the prevalence/risk of an event occurring
P event=no. people experiencing event/total no. people in group


what is relative risk?

how much more likely the event is to occur in one group compared to another


how do you calculate relative risk?

exposed group/risk of event in unexposed group


what does a risk ratio >1 indicate?

the risk of disease for the exposed group is larger than the risk of disease for the unexposed group


what other statistical measure is usually reported alongside risk?

confidence interval


what is relative risk difference/attributable risk?

1 - relative risk (when RR<1)


how do you calculate the absolute risk difference?

risk in exposed - risk in unexposed


what is the number needed to treat?

the additional number of people you would need to give a new treatment to in order to cure one extra person compared to old treatment


how is number needed to treat calculated?

calculated using absolute risk difference


how do you calculate odds?

p=probability of event occurring


how do you calculate odds ratio?

odds exposed group/odds unexposed group


what does it indicate if the odds ratio>1?

the odds of disease in the exposed group are larger than the odds of disease in the unexposed group, exposure to the factor increases the risk of contracting the disease


when will the odds ratio and relative risk be similar?

total sample is large and event is rare


what is absolute risk?

measure of risk of a certain event happening
more useful at communicating true impact


what is relative risk?

measure of the risk of a certain event happening in one group compared with the risk of the same event happening in another group (RR of 1 means there is no difference between groups


what type of study is best placed to answer questions about effectiveness of a particular therapy?

systemic reviews of RCT


What are the different kinds of RCT?

factorial trials
cluster trials


what is a factorial trial?

several therapies may be evaluated at same time by allocating pts to combinations of drugs


what is a cluster trial?

groups of patients are randomised (rather than individual patients being assigned to a particular Tx) i.e. all pts at one surgery/ on one ward


What type(s) of study design are best placed to answer questions about the aetiology of / risk factors for a condition?

cohort or Case-Control study


in what situation is a case control the only option?

If the condition under study is rare, or the time from exposure to development of the condition is particularly long, a case-control study may be the only realistic possibility, despite the drawbacks of case-control study methodology


What type(s) of study would be best suited to answering questions about the prognosis of a particular condition?



what questions are asked in a critical appraisal?

Are results of study valid?
What are results? Are they important?
Are results applicable to our patient/situation?


describe a randomised control trial?

qRandomised controlled trial recruits eligible participants then randomly allocates them to a treatment arm. It is very important that the allocation is done in a truly random manner, for example by computer-generated random numbers. There are other types of trial that are not randomised. It is preferable that wherever possible the treatment allocated is kept secret from both the participants and those assessing the results – this is called double blinding


Why is randomization the best allocation method for interventions?

1. Randomisation + allocation concealment used to minimize selection bias, to ensure Tx and control groups are comparable (groups similar across all patient characteristics and without significant differences at baseline).
2. This allows us to correctly identify + quantify Tx effects (or lack thereof)
3. Incomplete randomization leads to selection bias, where there are significant baseline differences between the 2 groups.
4. Any differences between groups at baseline leads us to over- or under- estimating the effect of the treatment (usually leads to over-estimation of Tx effect)


give an example of good randomisation?

truly random. E.g. done by computer-generated random numbers


give an example of an inadequate method of randomisation?

randomization based on date of presentation, order of presentation to clinician, letter of surname/number in date of birth etc.
These are inadequate methods as they may allow recruiting clinician to predict which treatment group the patients might be in, before deciding to recruit them into the study- leading to baseline differences between study groups


what are the consequences if the intervention and control group are different at baseline in RCT?

Any differences between treatment + control groups at baseline leads to us over- or under-estimating the treatment effect. Usually it leads to overestimation of treatment effectiveness.
This means we conclude there’s a significant difference in outcome between the two groups, where in fact there is not, this is known as Type 1 statistical error.


why is allocation concealment necessary in RCT?

Allocation concealment prevents clinicians performing the allocation from predicting what group patients would be in before deciding to recruit them to the trial.