Absolute risk reduction
Encyclopedia
In epidemiology
, the absolute risk reduction or risk difference is the decrease 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
.
For instance, consider a hypothetical drug which reduces the relative risk
of colon cancer by 50% over five years. Even without the drug, colon cancer is fairly rare, maybe 1 in 3,000 in every five-year period. The rate of colon cancer for a five-year treatment with the drug is therefore 1/6,000, as by treating 6,000 people with the drug, one can expect to reduce the number of colon cancer cases from 2 to 1.
In general, absolute risk reduction is usually computed with respect to two treatments A and B, with A typically a drug and B a placebo
(in our example above, A is a 5-year treatment with the hypothetical drug, and B is treatment with placebo, i.e. no treatment). A defined endpoint has to be specified (in our example: the appearance of colon cancer in the 5 year period). If the probabilities
pA and pB of this endpoint under treatments A and B, respectively, are known, then the absolute risk reduction is computed as (pB - pA).
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.
believe that this information is best presented as a natural number
in the context of the baseline risk ("reduces 2 cases of colon cancer to 1 case if you treat 6,000 people for five years"). Natural numbers, which are used in the number needed to treat approach, are easily understood by non-experts.
Epidemiology
Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...
, the absolute risk reduction or risk difference is the decrease in risk
Risk
Risk is the potential that a chosen action or activity will lead to a loss . The notion implies that a choice having an influence on the outcome exists . Potential losses themselves may also be called "risks"...
of a given activity or treatment in relation to a control activity or treatment. It is the inverse of the number needed to treat
Number needed to treat
The number needed to treat 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...
.
For instance, consider a hypothetical drug which reduces the relative risk
Relative risk
In statistics and mathematical epidemiology, relative risk is the risk of an event relative to exposure. Relative risk is a ratio of the probability of the event occurring in the exposed group versus a non-exposed group....
of colon cancer by 50% over five years. Even without the drug, colon cancer is fairly rare, maybe 1 in 3,000 in every five-year period. The rate of colon cancer for a five-year treatment with the drug is therefore 1/6,000, as by treating 6,000 people with the drug, one can expect to reduce the number of colon cancer cases from 2 to 1.
In general, absolute risk reduction is usually computed with respect to two treatments A and B, with A typically a drug and B a placebo
Placebo
A placebo is a simulated or otherwise medically ineffectual treatment for a disease or other medical condition intended to deceive the recipient...
(in our example above, A is a 5-year treatment with the hypothetical drug, and B is treatment with placebo, i.e. no treatment). A defined endpoint has to be specified (in our example: the appearance of colon cancer in the 5 year period). If the probabilities
Probability
Probability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we arenot certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The...
pA and pB of this endpoint under treatments A and B, respectively, are known, then the absolute risk reduction is computed as (pB - pA).
The inverse of the absolute risk reduction, NNT, is an important measure in pharmacoeconomics
Pharmacoeconomics
Pharmacoeconomics refers to the scientific discipline that compares the value of one pharmaceutical drug or drug therapy to another. It is a sub-discipline of health economics. A pharmacoeconomic study evaluates the cost and effects of a pharmaceutical product...
. If a clinical endpoint is devastating enough (e.g. death
Death
Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....
, heart attack
Myocardial infarction
Myocardial infarction or acute myocardial infarction , commonly known as a heart attack, results from the interruption of blood supply to a part of the heart, causing heart cells to die...
), drugs with a low absolute risk reduction may still be indicated in particular situations. If the endpoint is minor, health insurers
Health insurance
Health insurance is insurance against the risk of incurring medical expenses among individuals. By estimating the overall risk of health care expenses among a targeted group, an insurer can develop a routine finance structure, such as a monthly premium or payroll tax, to ensure that money is...
may decline to reimburse drugs with a low absolute risk reduction.
Presenting results
The raw calculation of absolute risk reduction is a probability (0.003 fewer cases per person, using the colon cancer example above). Authors such as Ben GoldacreBen Goldacre
Ben Michael Goldacre born 1974 is a British science writer, doctor and psychiatrist. He is the author of The Guardian newspaper's weekly Bad Science column and a book of the same title, published by Fourth Estate in September 2008....
believe that this information is best presented as a natural number
Natural number
In mathematics, the natural numbers are the ordinary whole numbers used for counting and ordering . These purposes are related to the linguistic notions of cardinal and ordinal numbers, respectively...
in the context of the baseline risk ("reduces 2 cases of colon cancer to 1 case if you treat 6,000 people for five years"). Natural numbers, which are used in the number needed to treat approach, are easily understood by non-experts.
External links
- Measures of effect size of an intervention - unmc.edu.