My simple ETA estimator

Here is an improved version of how we were calculating ETA in one of my previous company:

You need 4 scores as follow: 

• Effort(task) = Task effort team average point            (people agnostic) [1=easy;2=medium; 3=hard]
• Complexity(task) = Complexity effort team average point   [Fibonacci: 1=easy, 2=medium, 3=hard, 4=quite hard ...]
• Knowledge(task, user) = expertise/knowledge [1=expert;2=intermediate....]  (Bob+task#1=2; Alice+task#1=3)
• UserEfficiency(user) = user point/day factor [1,2,3...]             (ex: Bob=2; Alice=3)

Once you assign a user to a task, you can compute an ETA like this: 

ETA (task,user)= Effort(task)*Complexity(task)*Knowledge(task,user)*UserEfficiency(user)

example:

Effort(task1)=1

Effort(task1)=1

Complexity(bob,task1)=2

Complexity(alice,task1)=3

UserEfficiency(bob,task1)=2

UserEfficiency(alice,task1)=3

ETA(task1,Bob)=1*1*2*2=4 days

ETA(task1,Bob)=1*1*3*3=9 days