What are 2 difficulties in preparing accruals for long term employee benefits & What controls should be in place to support these accruals?

Usually accruals for long term employee benefits are not reasonable due to inappropriate assumptions,  Following factors may be helpful in accruals of employee benefits.

1) Stratification of employees based on salaries and ranks

2) Average salary of employees in each stratum

3) Average employees leave during the year in each stratum

4) interest rates calculation

5) Average increments in each stratum

6) Average people reach to retirement age

Past trends provides the basis in for making the said assumptions.