The bottom-box (also referred to as bottom-two-box or bottom-three-box scores) is a calculation method for scale questions. It is the sum of percentages for the lowest point(s), or least favorable point(s), on a satisfaction, intent or awareness scale. This calculation method is used as an alternative for the mean score for the scale. The idea behind this method is that you only take those responses that are expressing a strong attitude such as "extremely dissatisfied". 


Calculation: 

Count the number of respondents that selected the lowest, or least favorable, response and divide this count by the total number of responses. 


An example:

You have conducted a customer satisfaction research with a 5 point scale questions. The responses can be categorizes as follows:


120 out of 1000 (12%) have answered with Very satisfied

340 out of 1000 (34%) have answered with Satisfied

250 out of 1000 (25%) have answered with Neutral

190 out of 1000 (19%) have answered with Dissatisfied

100 out of 1000 (10%) have answered with Very dissatisfied


The bottom-two-box score is the combination of the two lowest categories. So you add up the number of respondents that have chosen either Very dissatisfied or Dissatisfied, in this case 290 respondents out of the total number of 1000 (=29%).


A lower bottom-box score means that a smaller part of your respondents is (very) negative in their answer.