Trying to determine attrition

Good day. I’m trying to determine what percentage of employees progress from each grade to the next and, more importantly, the attrition rates at each grade, but I have not heard back from HR with the numbers I seek. Instead, I have promotion statistics. I know how many people compete each year at each grade (and how many get promoted). I just can’t see how many retire or otherwise quit at each grade. I’m not interested in how they leave. I only want to know how many move up and how many depart.

Using a simplified grade structure, 1 being the lowest and 6 the highest, I’ll provide the numbers and ask if there is more to take into consideration than I am.

Let’s consider grade 1 to be a filter to determine who can progress toward a successful career. People could have a full 20 to 30 year career and depart at any of grades 2, 3, 4, 5 or 6. After a promotion, an employee must wait three years before being eligible to compete. Every employee who is eligible to compete does. No one starts above grade 1, they only get promoted to the upper grades from within.

Approximately 90 promote to 2 each year.
From grade 2 to 3, 500 compete and 50 are promoted.
From grade 3 to 4, 250 compete and 14 are promoted.
From grade 4 to 5 , 42 compete and 8 are promoted.
From grade 5 to 6, 20 compete and 2 are promoted.

I’m inclined to say that 250 employees never make it from 2 to 3 because 500 regularly compete from 2 to 3 and only 250 regularly compete from 3 to 4.

If my initial thought is correct, then approximately 4/5 of all grade 3 depart before making 4, while just under 20% make it to 4.

If that’s correct, can I also assume only 1/25 of all grade 2 employees make it to grade 5?

If I’m way off base, please let me know what I’m doing wrong and if possible, pass along a formula.

Thank you in advance,