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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 (non-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.

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 health-care 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 time-specific. 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):709-17.

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 angiotensin-converting-enzyme 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 follow-up 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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