In order to get the proportional_decrease (refer to documentation/json-schema/delta.json) credibility signal , first we need to have the proportional delta table.

proportional_decrease is the sum of negative values in the prop_delta_table (definition in revscoring code)

Example:

prop_delta_table = {'apple': -1/3, 'shoes':1, 'cat': -1, dog:1}

proportional_increase = -1/3 - 1 = -1.3333