Interpreting Graphs Flashcards
What does likelihood ratio nomogram show?
Pre-test probability
Likelihood Ratio
Post-test probability
What is the post-test probability?
Probability that a patient scoring positive on a test actually has the diseae
How does one get the post test probability from a likelihood nomogram?
Straight line drawn through pre-test probability and likelihood ratio will yield post-test probability
How does one obtain the total sample size needed to detect group difference (effect) with specified ability (power)?
Altman Nomogram
How does one draw an Altman Nomogram?
By joining value of power on right-hand axis with standardized difference (size of effect) on left-hand axis using a straight line
What does the intersection of the line with the upper part of the central platform in an Altman Nomogram give us?
The sample size required for the study to detect the difference with a preset probability threshold for false positives, p of 0.05
What does the intersection with the lower part of the central platform on an Altman Nomogram tell us?
Sample size required for study to detect difference with p of 0.01
How does one calculate the effect size from the Altman nomogram?
By the standardised difference
Calculation for standardised difference?
Target difference / standard deviation
What is the standardised difference referring to?
Target difference between 2 groups studies expressed in terms of the standard deviation of variable across the study population
How can one estimate the standard deviation before conducting a study?
Searching literature
Doing small pilot study
What may one wish to calculate before starting a study?
Required sample size by keeping constant p-value, power and estimate an expected effect size
How can one use Altmans nomogram for a study already completed?
Can test the power of the study using the given sample size, found effect size and set p-value
What does sample size depend on?
P value
Power
Expected difference
Expected variability of outcome variable in study population (SD)
What does funnel plot detect?
Publication bias
X and Y axis of a funnel plot?
X axis: measure of effect size
Y axis: measure of precision
What can be used as measure of effect size on funnel plot?
Cohens D
OR
RR
What can be used as measure of precision on funnel plot?
Sample size
Standard error
Where do studies with larger sample sizes go on a funnel plot?
Plotted higher on y axis
Where is true effect size obtained from?
Meta-analysis
What would we expect in the relationship between larger studies and the true effect size (drawn as a vertical line)?
Larger studies (at the top end) to be closer to the line
What would we expect in the relationship between smaller studies and the true effect size (drawn as a vertical line) on a funnel plot?
Smaller studies (at the lower end) would be spread from the line of true effect size
Why should larger studies be closer to the line of true effect size in funnel plots?
Because they are more representative and have results closer to the true effect sizes
How do we assume from a funnel plot that there is no publication bias?
Inverted funnel
