Before revision: where: is the standardised central exposed to risk from age x to x + t for the standard population, is the central exposed to risk from age x to x + t for the population under study and is the mortality rate in the standard population for ages x to x + t. The expression can be thought of as the crude mortality rate for the standard population divided by what the crude mortality rate is for the region being studied, assuming the mortality rates are the same as for the standard population.
After revision: where: is the standardised central exposed-to risk from age x to x + t for the standard population, is the central exposed-to risk from age x to x + t for the population under study and is the mortality rate in the standard population for ages x to x + t. The expression can be thought of as the crude mortality rate for the standard population divided by what the crude mortality rate is for the region being studied, assuming the mortality rates are the same as for the standard population.