When comparing groups, it is important to make sure we are making a fair comparison. Thus, it is helpful to standardize the rates, in order to remove the effect of a potential confounder (often age), which might differ between populations and could distort the results. Standardization is also helpful when comparing rates of one population over time, such as monitoring disease in a population over many years.
Disease can be measured in one population or compared between populations. Within one population, it is common to summarize disease burden with the number of cases. Another measure is the crude rate (i.e., x cases / y population atrisk), which you will also recognize as the cumulative incidence rate. If the distribution of a modifier of disease frequency (such as age) is different between two populations, however, a comparison of the crude rates in the two populations can mask the rate.
 A standardized rate is a measure of disease frequency that facilitates comparisons of populations with a different distribution of one or more potential confounding variables. (e.g., x cases / y population atrisk, adjusted to remove the effect of potential confounder [e.g., age]).
 Agespecific rates (i.e.., x cases in a specific age group/(population atrisk in same age group) are also useful in summarizing the health status of a population.
Types of Standardization Section
There are two different approaches to standardizing a rate:
Direct standardization
Direct standardization, more commonly used, creates a summary disease rate for a population that would be expected if the study population had a population distribution identical to that of an arbitrarily chosen standard population. A reference population is used as the standard population. The standardized rate is the sum of weighted groupspecific rates, with weights derived from the standard population. The weights sum to 1.0. A standardized rate is essentially a weighted average of agespecific rates.
\(I_{W}=\dfrac{\sum W_{i} I_{i}}{\sum W_{i}}\)
where I_{i }is a groupspecific rate and \(\sum W_{i}=1\)
The necessary data for direct standardization is the groupspecific disease rates for the study population and the population distribution from the standard population.
Stop and Think!
Do you understand how direct adjustment is a weighted average of agespecific rates?
Indirect standardization
Indirect standardization also produces a weighted average, through the production of a summary disease rate for the study population which would be expected if the disease experience of the study population were identical to that of a standard population. The standard population is arbitrarily chosen, but should be as similar as possible to the study population Indirect adjustment is used when accurate groupspecific rates for the study population are not available. If these rates are available, direct adjustment is preferred because it uses more information from the study population. Indirect adjustment produces an expected rate. Observed and expected rates are typically compared as a standardized ratio. Indirect adjustment is often used in occupational health to calculate standardized mortality ratios, which is dividing the observed death rate by the expected death rate.
The data required for indirect standardization is the crude rate for the study population; the population distribution for the study population and groupspecific rates for the standard population.
Example Section
Consider the below data for which the researcher could not obtain the genderspecific rates.
From a standard population, it is known that the crude rate is 1.5/1000, the male rate is 2.2/1000, and the female rate is 0.9/1000.
If we also know that:
 Group 1 male 60%, female 40%
 Group 2 male 80%, female 20%
What is the expected crude rate for group 1?
It is (2.2 × 0.6) + (0.9 × 0.4) = 1.68 / 1000. The observed crude rate is 1.68.
Stop and Think!
Come up with an answer to this question by yourself and then click on the button below to reveal the solution.
What is the expected crude rate for group 2?
Answer: the expected crude rate is (2.2 * 0.8) + (0.9 * 0.2) = 1.94.
Try it! Comparison of Direct and Indirect Standardization Methods Section
Try the following true/false questions to test your knowledge.

Ageadjusted rates are measures of mortality risk, conveying the magnitude of a health problem.
False

Ageadjusted rates can be compared regardless of the standard population used.
False
Direct standardized rates are only comparable if the same standard population is used. For example, the US standard population of 1940 was considerably younger than the US standard population based on the 2000 census. This will affect the adjusted rates. Always pay attention to the reference population when comparing standardized rates. 
Direct age adjustment is an average of the observed agespecific rates, weighting each agespecific rate by the proportion of that same age group in a standard population.
True

Indirect adjustment is preferred if there are only a few cases across all age groups.
True

If I want to understand the magnitude of a health problem, the appropriate statistic is the number of events.
True

To explore the underlying risk in a population, the appropriate statistic is a crude rate with its confidence interval.
True

To compare populations on the basis of differences in risk, after controlling for age, the appropriate statistic is the crude relative risk.
False
The ageadjusted rates and confidence intervals or relative risk (rate ratio) adjusted for age.