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 cancers 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.
Everitt, B.S. (2002) Cambridge Dictionary of Statistics, CUP.