Incidence Rate of Cancer
Age Standardized Incidence Rates per 100,000 world population
The Age standardized incidence rates are calculated first by estimating the agespecific rates and then applying these rates to the reference population, which is the standard world population. This process, called the direct standardisation is shown in table.
Computation of agestandardized incidence rates (stomach cancer, Denmark, males, 198892)

Age group i

No. of cases d_{i}

Persons at risk y_{i}

Standard world pop. w_{i}

Expected cases in standard pop. d_{i}/y_{i*}w_{i}

0 

0

749 800

12 000

0.00

5 

0

695 500

10 000

0.00

10 

0

808 900

9 000

0.00

15 

1

931 100

9 000

0.01

20 

2

1 017 500

8 000

0.02

25 

6

1 032 700

8 000

0.05

30 

4

955 800

6 000

0.02

35 

16

946 500

6 000

0.10

40 

34

1 025 500

6 000

0.20

45 

76

926 900

6 000

0.49

50 

97

718 900

5 000

0.68

55 

150

626 800

4 000

0.96

60 

187

590 800

4 000

1.27

65 

302

553 100

3 000

1.64

70 

315

449 900

2 000

1.40

75 

309

337 200

1 000

0.92

80 

247

196 200

500

0.63

85 +

152

115 700

500

0.66

Total

1 898

12 678 700

100 000

9.03

This table calculates the age standardized incidence rates for stomach cancer among Danish males, 198892. The standard world population is the reference population here.
Lets look at the table. d is the number of new stomach cancer cases, y is the number of persons at risk (given by multiplying the number of males in the different age groups in the Danish population by the observation period, 5 years in the case) and w is the number of persons in the different age groups per 100,000 standard world population.
Divide d by y for every age group and multiply that by w:
d/y_{*}w = age standardized incidence for that age group (0.49 for the 45 year old males)

Now the total of all the incidence rates gives you the Age standardized incidence rate/100,000 world population for stomach cancers among Danish males which is 9.03
Another example?
If one wants to calculate the Age standardized incidence rate of tongue cancer in the Algerian females 199799. You need simply the number of new tongue cancers in the different age groups of female during the 3 years (d in the table). You also take the population of Algerian females in the different age groups and multiply this by the number of years, in this case 3 (y in the table). Divide d by y for every age group.
Now you multiply this by the number of persons in the different age groups in the 100,000 standard world population (w in the table).
Add the incidence rates for the different age groups, the total is the Age standardized incidence rate for tongue cancers among the Algerian females in 199799.
Please note that if the numbers of cancers are little then the incidence rate can be expressed per 1 million standard world population instead of the more commonly used per 100,000  world population. This has been done in the Swedish studies of the Malignant Oral Tumours.
Annual Incidence Rates per 100,000 by age group
Again, let us look at the same table. d is the number of new stomach cancer cases, y is the number of persons at risk (given by multiplying the number of males in the different age groups in the Danish population by the observation period, 5 years in this case). Divide d by y for every age group and multiply that by 100,000.
d/y*100,000 = annual incidence rate of the cancer per 100,000 of that age group for that population

Source: Parkin D.M. et al. Cancer Incidence in Five Continents Vol. Vll , IARC Scientific Publications, No 143, Lyon, 1997.