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## Step 3:

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**Step 3:**Critically Appraising the Evidence: Basic Statistics**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 Table of Contents**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. Making It Easier**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.**Try it on your own.**• Critical Appraisal Practice Exercises • From CEBM**Links to Other Websitesand Hands-On Activities**• EBM Glossary • From CEBM • Critical Appraisal Practice Exercises • From CEBM**Congratulations!You have successfully completed Step 3:**Basic Statistics.The End