epidemiology Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and Risk factor (epidemiology), determinants of health and disease conditions in a defined population, and application of this knowledge to prevent dise ...

, the standardized mortality ratio or SMR, is a quantity, expressed as either a ratio or percentage quantifying the increase or decrease in mortality of a study cohort with respect to the general population.

Standardized mortality ratio

The standardized mortality ratio is the ratio of observed deaths in the study group to expected deaths in the general population. This ratio can be expressed as a percentage simply by multiplying by 100. The SMR may be quoted as either a

ratio In mathematics, a ratio () shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...

or a

percentage In mathematics, a percentage () is a number or ratio expressed as a fraction (mathematics), fraction of 100. It is often Denotation, denoted using the ''percent sign'' (%), although the abbreviations ''pct.'', ''pct'', and sometimes ''pc'' are ...

. 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. SMR constitutes an indirect form of standardization. It has an advantage over the direct method of standardization since age-adjustment is permitted in situations where age stratification may not be available for the cohort being studied or where strata-specific data are subject to excessive random variability.

Definition

The requirements for calculating SMR for a cohort are: * The number of persons in each age group in the population being studied * The age specific death rates of the general population in the same age groups of the study population * The observed deaths in the study population Expected deaths would then be calculated simply by multiplying the death rates of the general population by the total number of participants in the study group at the corresponding age group and summing up all the values for each age group to arrive at the number of expected deaths. The study groups are weighted based on their particular distribution (for example, age), as opposed to the general populations's distribution. This is a fundamental distinction between an indirect method of standardization like SMR from direct standardization techniques. The SMR may well be quoted with an indication of the

uncertainty Uncertainty or incertitude refers to situations involving imperfect or unknown information. It applies to predictions of future events, to physical measurements that are already made, or to the unknown, and is particularly relevant for decision ...

associated with its estimation, such as a confidence interval (CI) or p value, which allows it to be interpreted in terms of

statistical significance In statistical hypothesis testing, a result has statistical significance when a result at least as "extreme" would be very infrequent if the null hypothesis were true. More precisely, a study's defined significance level, denoted by \alpha, is the ...

Example

An example might be a

cohort study A cohort study is a particular form of longitudinal study that samples a Cohort (statistics), cohort (a group of people who share a defining characteristic, typically those who experienced a common event in a selected period, such as birth or gra ...

into cumulative exposure to

arsenic Arsenic is a chemical element; it has Symbol (chemistry), symbol As and atomic number 33. It is a metalloid and one of the pnictogens, and therefore shares many properties with its group 15 neighbors phosphorus and antimony. Arsenic is not ...

from

drinking water Drinking water or potable water is water that is safe for ingestion, either when drunk directly in liquid form or consumed indirectly through food preparation. It is often (but not always) supplied through taps, in which case it is also calle ...

, whereby the mortality rates due to a number of

cancer Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Po ...

s in a highly exposed group (which drinks water with a mean arsenic concentration of, say 10 mg) is compared with those in the general population. An SMR for

bladder cancer Bladder cancer is the abnormal growth of cells in the bladder. These cells can grow to form a tumor, which eventually spreads, damaging the bladder and other organs. Most people with bladder cancer are diagnosed after noticing blood in thei ...

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).

Standardized mortality rate

''Standardized mortality rate'' tells how many persons, per thousand of the population, will die in a given year and what the causes of death will be. Such statistics have many uses: *

Life insurance Life insurance (or life assurance, especially in the Commonwealth of Nations) is a contract A contract is an agreement that specifies certain legally enforceable rights and obligations pertaining to two or more parties. A contract typical ...

companies periodically update their premiums based on the

mortality rate Mortality rate, or death rate, is a measure of the number of deaths (in general, or due to a specific cause) in a particular Statistical population, population, scaled to the size of that population, per unit of time. Mortality rate is typically ...

, adjusted for age. * Medical researchers can track disease-related deaths and shift focus and funding to address increasing or decreasing risks. * Organizations, both non- and for-profit, can utilize such statistics to justify their missions. * Regarding occupational uses: Mortality tables are also often used when numbers of deaths for each age-specific stratum are not available. It is also used to study mortality rate in an occupationally exposed population: Do people who work in a certain industry, such as mining or construction, have a higher mortality than people of the same age in the general population? Is an additional risk associated with that occupation? To answer the question of whether a population of miners has a higher mortality than we would expect in a similar population that is not engaged in mining, the age-specific rates for such a known population, such as all men of the same age, are applied to each age group in the population of interest. This will yield the number of deaths expected in each age group in the population of interest, if this population had had the mortality experience of the known population. Thus, for each age group, the number of deaths expected is calculated, and these numbers are totaled. The numbers of deaths that were actually observed in that population are also calculated and totaled. The ratio of the total number of deaths actually observed to the total number of deaths expected, if the population of interest had had the mortality experience of the known population, is then calculated. This ratio is called the standardized mortality ratio (SMR). The SMR is defined as follows: SMR = (Observed no. of deaths per year)/(Expected no. of deaths per year).

References

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