Study design and summarising data Flashcards
what is a RCT
intervention study where subjects are randomly allocated to treatment options
What are the treatment groups compared with in RCT
placebo or uussual care
What does randomisation in RCT allow for
subjects characteristics don’t affect which treatment they receive=unbiased
to balance treatment groups by subject characteristics to make sure differences arissing can be attributed to being caused by treatment alone
fair test of efficacy, allows for blinding
what does blinding allow
concealing the allocation. Can be double or single blind
What is ITT and what does it achieve
attributing subject to the group they were originally allocated regardless to whether they change treatment plan.
You can be certain about balance of treatment groups with respect to characteristics of subjects so provides unbaised comparison of subjects
What is per protocol analysis and what negatives does this have, when is it useful
patient analysed according to treatment actually received
negative: balance in patient characteristics present after random allocation is lost.
useful: can be used with ITT if patient stopped/changed treatment
what is the limitation of case control study design
choice of control group affects comparison between case and control
exposure to risk factor usually collected reterospectively and may be incomplete, inaccurate or biased
How does cohort studies differ with case control study
longitudinal and starts with unselected group of individuals who are followed up for set period of time. Used to confim finding of case control
Cohort study design
unselecteed group of healthy individuals->followed up to monitor disease and potential risk factors
prospective, risk factor of disease recorded before disease is confirmed
can be retrospective but requires that full risk factor data is obtained on all individuals with and without disease of interest using data which was recorded prospectively
Difficulty with cohort studies
large no. subjects needed to obtain enough individuals who get disease or conditon particularly if uncommon
length of follow up may be substantial to get enough diseased individuals so cohort study is not feasible for rare diseases
difficulty in maintaining contact with subjects if lengthy follow up
resources required may be high
Cross sectional study what
sample chosen and data on each individual collected at one point in time. Could be different time points for each subject
when is cross sectional study used
survey of prevalence, attitudes/views, inter relationships between variables
why summarise data
data quality monitoring
data checking and data cleaning
baseline data in study
before doing complex analysis
what is ordinal data
data arranged in numerical order (can be quantitative or categorical data)
consequences of categorising continuous data
dichotomising (recategorizing in 2 groups) could be problematic because info and statistical power lost in analysis. Masks nature of relationships
grouping into several groups, stat power effect less than dichotomising. No problem is summary statistic and original data used in analysis
grouping can be useful in nonlinear relationships
what is the arithemetic mean
sum of all values/total no. values
what does standard deviation show
how dispersed the data is
how is variance calculated
summing the squared differences between overall mean and each value and dividing by no.values-1
why is standard deviation better than variance
easier to calculate, same unit as original data
What types of measures for centre of data do you use for: contin data with symmetrical distribution, continuous with positively skewed distribution, continuous data with skewed distribution, discrete data
symmetrical distribution-arithmetic mean
continuous with positively skewed distribution-geometric/harmonic mean
continuous data with skewed distribution-median
discrete data-median unless range of data is large and calculation of mean is sensible
what method to use for spread of distribution
continuous-standard deviation
continuous with skew-IQR
What to include in summary of nominal and ordinal data
nominal-freq and%
ordinal-freq, %, cumulative freq, cumulative %
How to spot skew
positive-tail on right is longer
negative-tail on left is longer
