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
How do we note the presence of publication bias in a funnel plot?
An obvious void at either corner of the funnel plot, suggestive of missing studies
In a ROC curve, what does the point at the start of the curve indicate?
Cut-off with lowest sensitivity
In a ROC curve, what does the point at the end of the curve indicate?
Cut-off with lowest specifcity
What does is the straight line made between the 2 points of a curve in a ROC curve called?
Chance line
What does a chance line on a ROC curve indicate?
Curve of a test that performs no better than diagnosing by chance
What is the leaf in the stem and leaf plot?
X axis
What is the stem in stem and leaf plot?
Y axis
Significance of leaf in stem and leaf plot
Each number corresponds to a single observation when the tenth difit value og the stem is added to the leaf value
How do identify endogenous variables in path analysis
The variables will have arrows originating from and pointing towards them
What is the beta regressor in path analysis?
Path coefficient
What is the R square in path analysis
Coefficient of determination
What is the disturbance term in path analysis
Error in measurement
When might log values be used in a scatter plot?
To yield parametric transformation
Calculation for precision
1/SE
X and Y axis of Galbraith plot
X: precision
Y: Effect size
What lines are shown on Galbraith plot?
Central line: line depicting pooled effect size
Line above: +2Sd line
Line below: -2SD line
How does one identify heterogeneity in a Galbraith plot?
Outliers from the lines
X and Y axis in L’Abbe plot
X: control group
Y: Intervention group
What are the lines in a L’Abbe plot?
Dotted
Equal
What is the dotted line in L’Abbe plot?
Pooled RR line - summarised RR from meta-analysis
What is the equal line in L’Abbe Plot?
Indicates that the event rates are equal in two arms within trial (RR=1)
What does it mean if a trial is below the equality line in L’Abbe plot?
Control groups have better event rates than intervention group
What does each circle represent in L’Abbe plot?
Individual trial
What does the size of the circle in L’Abbe trial correspond with?
Number of participants
What is the upper adjacent in box whisker plot?
Maximum value excluding outliers
What is the lower adjacent in box whisker plot?
Minimum value excluding outliers
What is the upper quartile in box whisker plot?
Corresponds to 75th percentile value
What is the lower quartile in box whisker plot?
Corresponds to 25th percentile value
What is the horizontal line in box whisker plot?
50th percentile - median
What does a dashed horizontal line mean in box whisker plot?
Mean
What are asterisks used for in box whisker plot?
Extreme outliers - >2SD or 3 box lengths away from quartiles
