The likelihood of an event happening under specific conditions (usually a “1/10 chance” or “10% chance”)
# of events (good or bad) in treated or control groups, divided by the number of people in that group
The likelihood of an event occurring in a group of people compared to another group with different behaviors, physical conditions, or environments (usually a % increase/decrease compared to groups)
RR = ART/ARC (# events in treatment/# events in control)
The ratio of the probability of an event occurring in the exposed group versus the probability of the event occurring in the nonexposed group
Shows exact difference from baseline, how much lower the modified risk is from the start risk in absolute terms
ARR = ARC – ART (# events in control  # events in treatment)
Tells us how much lower the modified risk is than the starting risk as a proportion. Can look very impressive for small differences.
Calculated RRR = 1RR OR (ARC – ART)/ARC
HR is a comparison between the probability of events in a treatment group compared to the probability of events in a control group
Absolute Risk Reduction, Relative Risk Reduction, Relative Risk resources
Compares odds of an exposure in cases to odds of exposure in controls
Is also a measure of outcome vs exposure
Odds Ratios resources
The number of patients you need to treat to prevent one additional bad outcome (death, stroke, heart attack, etc.)
NNT = 1/ARR
PPV = # of true positives / (# of true positives + # false positives) (top of 2x2 table)
NPV = true negative rate / (true negative rate + false negative rate)*100 (bottom of 2x2 table)
Sensitivity, Specificity and Likelihood ratios resources
Assess the potential utility of a particular diagnostic test & looks at how likely it is that a patient has a disease or condition
Understanding the numbers:
LR resources
What are likelihood ratios and how are they used Terry Shaneyfelt video (10 min)
measures extent of heterogeneity. Low is good, high is bad for numbers. >50% is a bad combination of tests.
Resources:
What is heterogeneity and is it important? BMJ 2007; 334 :94. Fletcher, J.
Deeks JJ, Higgins JPT, Altman DG (editors). Chapter 9: Analysing data and undertaking metaanalyses. In: Higgins JPT, Green S (editors). Cochrane Handbook for Systematic Reviews of Interventions Version 5.1.0 [updated March 2011]. The Cochrane Collaboration, 2011. Available from www.cochranehandbook.org.
What is Heterogeneity and why does it matter? Students 4 Best Evidence
Emily Shohfi, MLIS, AHIP
Clinical Librarian; EBM Course Facilitator
3012952603
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emily.e.shohfi.civ@mail.mil
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Trip (Turning Research Into Practice) is a database that only searches sites that have evidencebased content.
Helps decision makers identify the best available evidence by providing guides to the systematic consideration of the validity, importance, and applicability of claims about the assessment of health problems and the outcomes of health care. Includes the Users' Guide to the Medical Literature and the Rational Clinical Examination, among other resources.
The Cochrane Library is a collection of six databases containing different types of highquality, independent evidence to inform healthcare decisionmaking including:
Probability that the null hypotheses gives for a specific experimental result to happen; goal of experiments is to disprove the null hypothesis
95% confidence interval is a range of values that you can be 95% certain contains the true mean of the population.
Confidence intervals Terry Shaneyfelt video (5 min)
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