**Glossary**

**Absolute Risk Difference** is the arithmetic
difference between the rates of events in the intervention
and control group.

**Absolute Risk Reduction** refers to the decrease
of a bad event as a result of the intervention.

**Absolute Benefit Increase** refers to the increase
of a good event as the result of the intervention.

**Confidence Intervals** are calculated on the
results of the data to show the strength or weakness
of the evidence. A 95%CI[range] means that if you
were to repeat the same clinical trial a hundred
times you can be 95% sure that the data would fall
within the calculated range.

**Intention to treat** analysis of patients with
the treatment group to which they were originally
assigned, regardless of whether or not they actually
received the treatment or not.

**Likelihood Ratio** indicates the likelihood
that a given test result would be expected in a patient
with the target disorder compared to the likelihood
that the same result would be expected in a patient
without that disorder.

**Numbers Needed to Treat (NNT)** the number
of patients who need to be treated to prevent one
bad outcome. The NNT is a useful number when you
want to compare the costs and adverse effects of
a treatment with its benefits.

**Odds Ratio** describes the odds of an experimental
patient suffering an adverse event relative to a
control patient.

**P Value** refers to the probability that any
particular outcome would have arisen by chance. (The
smaller the P value the less likely the data was
by chance.) Standard scientific practice, usually
deems a P value of less than 1 in 20 (expressed as
P=.05) as "statistically significant". The smaller
the P value the higher the significance. A P value
of P=.01 ( less than 1 in 100) is considered "statistically
highly significant".

**Predictive Value of tests: **In screening and
diagnostic tests, the probability that a person with
a positive test is a true positive (i.e., has the
disease), is referred to as the positive Predictive
Value; whereas, the Negative Predictive Value is
the probability that the person with a negative test
does not have the disease. Predictive value is related
to the sensitivity and specificity of the test.

**Relative Risk** is the risk of developing a
disease in the exposed group divided by the risk
of developing the disease in the unexposed group.

**Relative Risk Reduction** is the proportional
difference between the rates of events in the control
group and the intervention group. Relative Risk Reduction
is usually a larger number than the Absolute Risk
Difference and therefore tends to exaggerate the
difference.

**Sensitivity** measures the proportion of patients
with the disease who also test positive for the disease.

**Specificity** measures the proportion of patients
without the disease who also test negative for the
disease.

A good test is both highly sensitive and highly
specific.

See also the Glossary
of Terms from Evidence Based Emergency Medicine and the New
York Academy of Medicine |

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