To find probability of achievine the goal

Hi,

How do we find the probability  of achieving the goal in the organiztion?what parameters or inputs do we require?

For example in context of Defect Density:

If the historical data (past data) of company  indicates the average 5 defects per person month and the goal is to have 2 defects per person month then what would be the probability of achieving the goal?

Regards,

Nandita R.