The principal actuarial assumptions used in determining the defined benefit obligation for the plans are as follows:
| December 31, | ||||||||
| 2017 | 2016 | |||||||
|
Rate of increase in salaries |
4.77 | % | 4.77 | % | ||||
|
Inflation assumption |
3.75 | % | 3.75 | % | ||||
|
Discount and return on plan assets rate |
7.89 | % | 8.17 | % | ||||
|
Average length of obligation (years) |
18.40 | 17.67 | ||||||