••• CHAPTER 64^ Basic Epidemiology and Clinical Biostatistics^649
❍ What is a control event rate (CER)?
The number of subjects in a control group who develop the outcome being studied.
❍ What is an experimental event rate (EER)?
The number of subjects in the experimental or treatment group who develop the outcome being studied.
❍ Define relative risk reduction (RRR).
RRR is the reduction in risk with a new therapy; it is the absolute value of the difference between EER and the
CER divided by the CER.
❍ Define absolute risk reduction (ARR).
Reduction in risk with a new therapy compared with the risk without a new therapy. It is the absolute value of the
difference between the EER and the CER.
❍ Define absolute risk increase.
The increase in risk with a new therapy compared with the risk without the new therapy.
❍ What is meant by a hazard ratio?
Ratio of risk of an outcome (such as miscarriage) occurring at any time in one group compared with another
group.
❍ What does NNT stand for?
NNT stands for the number needed to treat. NNT is the number of patients who need to be treated with a
proposed therapy in order to prevent, or cure one person. It is the reciprocal of the absolute risk reduction
(1/ARR).
❍ What does NNH stand for?
NNH stands for number needed to harm. NNH is the number of patients who need to be treated with a proposed
therapy in order to cause one undesirable outcome (1/ARI).