Standardized mortality ratio
Encyclopedia
The standardized mortality ratio or SMR in epidemiology
is the ratio of observed deaths to expected deaths, where expected deaths are calculated for a typical area with the same age and gender mix by looking at the death rates for different ages and genders in the larger population.
The SMR may be quoted as either a ratio
or a percentage
. If the SMR is quoted as a ratio and is equal to 1.0, then this means the number of observed deaths equals that of expected cases. If higher than 1.0, then there is a higher number of deaths than is expected.
An example might be a cohort study
into cumulative exposure to arsenic
from drinking water
, whereby the mortality rates due to a number of cancer
s in a highly exposed group (which drinks water with a mean arsenic concentration of, say 10mg) is compared with those in the general population. An SMR for bladder cancer
of 1.70 in the exposed group would mean that there is 70% more cases of death due to bladder cancer in the cohort than in the reference population (in this case the national population, which is generally considered not to exhibit cumulative exposure to high arsenic levels).
The SMR may well be quoted with an indication of the uncertainty
associated with its estimation, such as a confidence interval
(CI) or p value, which allows it to be interpreted in terms of statistical significance
.
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...
is the ratio of observed deaths to expected deaths, where expected deaths are calculated for a typical area with the same age and gender mix by looking at the death rates for different ages and genders in the larger population.
The SMR may be quoted as either a ratio
Ratio
In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...
or a percentage
Percentage
In mathematics, a percentage is a way of expressing a number as a fraction of 100 . It is often denoted using the percent sign, “%”, or the abbreviation “pct”. For example, 45% is equal to 45/100, or 0.45.Percentages are used to express how large/small one quantity is, relative to another quantity...
. If the SMR is quoted as a ratio and is equal to 1.0, then this means the number of observed deaths equals that of expected cases. If higher than 1.0, then there is a higher number of deaths than is expected.
An example might be a cohort study
Cohort study
A cohort study or panel study is a form of longitudinal study used in medicine, social science, actuarial science, and ecology. It is an analysis of risk factors and follows a group of people who do not have the disease, and uses correlations to determine the absolute risk of subject contraction...
into cumulative exposure to arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
from drinking water
Drinking water
Drinking water or potable water is water pure enough to be consumed or used with low risk of immediate or long term harm. In most developed countries, the water supplied to households, commerce and industry is all of drinking water standard, even though only a very small proportion is actually...
, whereby the mortality rates due to a number of cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
s in a highly exposed group (which drinks water with a mean arsenic concentration of, say 10mg) is compared with those in the general population. An SMR for bladder cancer
Bladder cancer
Bladder cancer is any of several types of malignant growths of the urinary bladder. It is a disease in which abnormal cells multiply without control in the bladder. The bladder is a hollow, muscular organ that stores urine; it is located in the pelvis...
of 1.70 in the exposed group would mean that there is 70% more cases of death due to bladder cancer in the cohort than in the reference population (in this case the national population, which is generally considered not to exhibit cumulative exposure to high arsenic levels).
The SMR may well be quoted with an indication of the uncertainty
Uncertainty
Uncertainty is a term used in subtly different ways in a number of fields, including physics, philosophy, statistics, economics, finance, insurance, psychology, sociology, engineering, and information science...
associated with its estimation, such as a confidence interval
Confidence interval
In statistics, a confidence interval is a particular kind of interval estimate of a population parameter and is used to indicate the reliability of an estimate. It is an observed interval , in principle different from sample to sample, that frequently includes the parameter of interest, if the...
(CI) or p value, which allows it to be interpreted in terms of statistical significance
Statistical significance
In statistics, a result is called statistically significant if it is unlikely to have occurred by chance. The phrase test of significance was coined by Ronald Fisher....
.
Resources
- PAMCOMP Person-Years Analysis and Computation Programm for calculating SMRs