Where did you get that number? And "20% reduction" is meaningless without saying 20% of what. Like, 20% reduction of a 1/1000000 chance is not the same as a 20% reduction of a 1/3 chance.

There are two common fallacies I see here. The first happens a lot with interpreting carcinogenicity or toxicity studies, where an X% increase/decrease is quoted without an absolute value. The second happens in the stereotypical startup pitch of "if only we could get just 20% of that market" -- it's a made-up number with no basis for why that is the right number.

It is a completely made up number. If your argument against increased surveillance is based on its ineffectiveness, then there has to be a level of effectiveness where you would agree that that it's acceptable.

> If your argument against increased surveillance is based on its ineffectiveness, then there has to be a level of effectiveness where you would agree that that it's acceptable.

Not necessarily. Someone could be try to persuade by offering an argument that he thinks will be appeal to some, but does not represent his full thinking on the subject (e.g. he could also have moral reasons for opposing increased surveillance).