Transcript Evidence-Based Medicine

Step 3:
Critically Appraising
the Evidence:
Basic Statistics
Table of Contents
• Clinical Statistics Calculator (Excel)
• Statistics for:
– General
• Samples and Populations
• Means and Medians
• Normal and Skewed Distributions
• Variability, Variance, and Standard Deviation
• Confidence Intervals
• p-values
• Generalized 2x2 Clinical Table
• Practice Exercises
Making It Easier
•If available, find the best evidence in
secondary sources where analysis has
already occurred.
•If not pre-assessed, use critical
appraisal worksheets to help you
through the process.
Importance of Critically
Appraising the Evidence
• Understanding the Limitations of the
Author’s Analyses and Interpretations of
the Data
• Assessing Internal Validity
• Assessing External Validity
• Identifying Potential Confounding Variables
– Simpson’s Paradox
Critical Appraisal Basics
• View movie as:
– QuickTime
(.mov)
– Flash (.swf)
•
Double-click on video for full-screen mode.
Population vs. Sample
• Population
– The entire group of all potential patients
• Sample
–
–
–
–
The patients included in the study
Representative of the population
Clinical samples consist of treatment groups
Non-clinical samples consist of control groups
Basic Statistical Tools:
Mean
• Often called arithmetic average or just average
• Sum of all of the data points divided by the
number of data points
Basic Statistical Tools:
Median
• If there are n data points lined up in numerical
order the median is the one in the exact middle
or the average of the 2 middle points if there is an
even number of data points. In other words it is
the nth/2 data point if n is odd and the ((n-1)th/2 +
(n+1)th/2)/2 data point if n is even.
Median:
In Layman’s Terms
– Line up all of the data points in increasing order.
– The one in the middle is the median.
– If there is no clear single mid-point (i.e. there is an
even number of data points), the median is half-way
between the two middle points.
• So if 0, 1, 2, 4 were our data set, 1.5 would be the median.
Normal vs. Skewed
• Normal
– Symmetric, bell-shaped
distribution where the
frequency of data within
an interval is greater the
nearer it is to the mean
• Skewed
– Asymmetric distribution
– More data to one side of
the mean than the other
Mean vs. Median
• Additional outliers
– usually increase/decrease mean more than
median
• Skewed data
– Mean shifted toward the tail (i.e. the side where
the data points are more spread out)
– Median unaffected by distribution/shape
•Always the middle value regardless of where the
other points lie
When is the distinction between
mean and a median important?
• Mean (i.e. Arithmetic Average)
– Used when data is approximately normally distributed
• Median
– Sometimes used with skewed data due to its robustness
– Preferred on Likert scales (survey data) since values are
ranked but their differences are not clearly quantifiable.
• After all, what number does “agree” minus “disagree” equal?
Regardless of how one quantifies these differences, the median
remains the same, but the mean depends on the scale.
If mean and median greatly differ
• either:
– the data is not normal and is skewed one way
or another
– there is at least one outlier with a lot of leverage
Basic Statistical Tools:
Measures of Variability
• Variability / Dispersion
– How data points are distributed
• Variance
– Sum of the squares of the
differences between each data
point and the sample mean,
divided by the total number of
data points
• Standard Deviation
– Square root of the variance
Confidence Intervals
• Range in which one would expect the
values of the outcome variables to fall into
when replicating the experiment given a
quantifiable probability of error.
p-value
• Probability of obtaining a result as extreme
as the observed one if the data were a result
of chance.
• Small p-values mean the result could not
likely be the result of chance.
• Often researches use p-values such as 0.10,
0.05, 0.01, and smaller.
Generalized 2x2 Clinical Table
Outcome of
Intervention/
Comparison
(Alternatives)
+
-
Actual Outcome
+
a (True
b (False
Positives)
Positives)
c (False
d (True
Negatives)
Negatives)
Try it on your own.
• Critical Appraisal Practice Exercises
– From CEBM
Links to Other Websites
and Hands-On Activities
• EBM Glossary
– From CEBM
• Critical Appraisal Practice Exercises
– From CEBM
Congratulations!
You have successfully
completed Step 3:
Basic Statistics.
The End