It is a method to evaluate surveys especially Tankans.
The best way to explain how it works is an example: Assume that 70 of 145 asked persons answer "yes" and 75 fill in "no", that would mean that the positive ones make 48% and the negative ones 51%. Then they calculate: (+48) + (-51) = -3. Finally this result is in the table or chart, or wherever the survey is published.