Hi Sonya:

there is not a "formula", but in general the efficiency (or performance) decrease exponentially (not linear), and the cost is increased exponentially.

This is due several factors:

• overtime (man-hours) are more expensive
• overtime has a natural loss of efficiency (for human resources)
• additional shifts (night shifts) have the same problems (above)
• contribution of additional resources (people) finally is inefficient (due the "decreasing performance law). In fact, too much additional resources will block the progress
• additional incentive payments will be requested
• additional supervision is required

If your problem is to recover a delay or just to decrease the total time (through the critical path), the are 2 options: fast track or crashing time.

For details, see PMBOK or other (exists a large literature).

Regards

(note: the picture is not mine, but it is a classical)

chart01.png