How to calculate Test Design Productivity Improvement ?

Test Design Productivity Improvement is the ratio of difference between test design productivity of current year and last/prior year to test design productivity of last year. If the Test Case Design Productivity is low due to changes in the requirement, it helps to factor the effort into the project plan and re-estimate the project plans. 
Test Design Productivity Improvement is measured in percentage (%).

How to calculate Test Design Productivity Improvement:

Test Design Productivity improvement = (Test Case productivity of current year - Test Case productivity of last year) / (Test Case productivity of last year) * 100

Example:

Test Design  productivity of last year	Test Design productivity of current year	Test Design Productivity improvement
8	10	25

Test Design Productivity Improvement = (10-8) / 8 *100

                                                               = 25 %