Adjusting rates to take into account how many old or young people are in the population being studied. When rates are age standardised, differences in the rates over time or between geographical areas do not simply reflect variations in the age distribution of the populations. This is important when looking at cancer because it is a disease that predominantly affects the elderly. So if cancer rates are not age standardised, a higher rate in one country is likely to reflect the fact that it has a greater proportion of older people.
A