Percentages » Index numbers
What are index numbers?
A percentage that is relative to a base value is called an index number. Behind the index number you do not have to write a %-symbol.
Often historical date is compared over a couple of years. The base value corresponds to a certain year, this year is called the base year.
Example
In the table, the year 2015 has been chosen as the base year.
The value of 2015 is therefore the base value and is 100%.
The index numbers can be found in the last column.
| Year | Value | In- or decrease compared to 2015 in % | Index number |
| 2012 | 53 412 | –9.1 | 1 90.9 |
| 2013 | 53 726 | –8.5 | 1 91.5 |
| 2014 | 56 243 | –4.3 | 1 95.7 |
| 2015 | 58 743 | –0 | 100 |
| 2016 | 61 467 | –4.6 | 104.6 |
| 2017 | 59 551 | –1.4 | 101.4 |
Calculating with index numbers
Because an index number is a percentage, you can calculate these the same way.
Example 1
See the table above.
The value that corresponds to 2018 is 60 342.
Calculate the index number for the year 2018.
Answer:
60 34258 743 × 100 ≈ 102.7
Example 2
See the table above.
It is known that the index number of the year 2011 is 89.7.
Calculate the value for the year 2011.
Answer:
89.7 : 100 × 58 743 ≈ 52 692