NNT (Number needed to treat) Calculator

Control Group

Number of patients in placebo arm that had the outcome of interest:

Total number of patients in placebo group 
n(total):

CER = CONTROL EVENT RATE: percent of patients in the control or placebo arm
that had the outcome of interest. Formula:
CER = [# of pts in placebo arm that had the outcome of interest] / [total #
of patients in placebo group]
In epidemiology and biostatistics, the control event rate (CER) is a measure
of how often a particular statistical event (such as response to a drug,
adverse event or death) occurs within the scientific control group of an
experiment. This value is very useful in determining the therapeutic benefit
or risk to patients in experimental groups, in comparison to patients in
placebo or traditionally treated control groups.

Experimental Group

Number of patients in the experimental arm that had the outcome of interest:

Total number of patients in the experimental group 
n(total):

EER = EXPERIMENTAL EVENT RATE: % of patients in the experimental arm that
had the outcome of interest. Formula: EER = [# of pts in
experimental arm that had the outcome of interest] / [total # of patients in
experimental group]
In epidemiology and biostatistics, the experimental event rate (EER) is
a measure of how often a particular statistical event (such as response to a
drug, adverse event or death) occurs within the experimental group
(noncontrol group) of an experiment. This value is very useful in
determining the therapeutic benefit or risk to patients in experimental
groups, in comparison to patients in placebo or traditionally treated
control groups.


Other Definitions

ABSOLUTE RISK REDUCTION (ARR)

ABSOLUTE RISK REDUCTION (ARR): (absolute value of the
difference between the CER and the EER ) Formula: CER  EER 
or = 1/NNT
In epidemiology, the absolute risk reduction, risk difference or excess
risk is the change in risk of a given activity or treatment in relation to a
control activity or treatment. It is the inverse of the number needed to treat
(NNT).

NUMBER NEEDED TO TREAT (NNT)

NNT = Number of patients that must be given the experimental treatment for the
duration of the study to prevent a single outcome e.g. death or other
measurable variable. Formula: NNT = 1/ARR
The inverse of the absolute risk reduction, NNT, is an important measure in
pharmacoeconomics. If a clinical endpoint is devastating enough (e.g. death,
heart attack), drugs with a low absolute risk reduction may still be indicated
in particular situations. If the endpoint is minor, health insurers may
decline to reimburse drugs with a low absolute risk reduction.
The number needed to treat (NNT) is an epidemiological measure used in
assessing the effectiveness of a healthcare intervention, typically a
treatment with medication. The NNT is the average number of patients who need
to be treated to prevent one additional bad outcome (i.e. the number of
patients that need to be treated for one to benefit compared with a control in
a clinical trial). It is defined as the inverse of the absolute risk
reduction. It was described in 1988. The ideal NNT is 1, where everyone
improves with treatment and no one improves with control. The higher the NNT,
the less effective is the treatment.
NNT values are timespecific. For example, if a study ran for 5 years and it
was found that the NNT was 100 during this 5 year period, in one year the NNT
would have to be multiplied by 5 to correctly assume the right NNT for only
the one year period (in the example the one year NNT would be 500).

RELATIVE RISK REDUCTION (RRR)

RELATIVE RISK REDUCTION (In epidemiology, the relative risk reduction is a
measure calculated by dividing the absolute risk reduction by the control
event rate.)
Formula: RRR (raw calculation) = CER
 EER / CER
[ Alt: RRR = 1 RR where RR=reported relative risk]
The two methods of calculations may produce different results. The reported
value will adjust for other prognostic factors. 
Sample  Filling in the form

Source:
Pitt B, Zannad F, Remme WJ, Cody R, Castaigne A, Perez A, Palensky J, Wittes J.
The effect of spironolactone on morbidity and mortality in patients with severe
heart failure. Randomized Aldactone Evaluation Study Investigators. N Engl J
Med. 1999 Sep 2;341(10):70917.
Abstract
BACKGROUND AND METHODS:
Aldosterone is important in the pathophysiology of heart failure. In a
doubleblind study, we enrolled 1663 patients who had severe heart failure and a
left ventricular ejection fraction of no more than 35 percent and who were being
treated with an angiotensinconvertingenzyme inhibitor, a loop diuretic, and in
most cases digoxin. A total of 822 patients were randomly assigned to receive 25
mg of spironolactone daily, and 841 to receive placebo. The primary end point
was death from all causes.
RESULTS:
The trial was discontinued early, after a mean followup period of 24 months,
because an interim analysis determined that spironolactone was efficacious.
There were 386 deaths in the placebo group (46 percent) and 284 in the
spironolactone group (35 percent; relative risk of death, 0.70; 95 percent
confidence interval, 0.60 to 0.82; P<0.001). This 30 percent reduction in the
risk of death among patients in the spironolactone group was attributed to a
lower risk of both death from progressive heart failure and sudden death from
cardiac causes.

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