A few bad apples

I always wondered how many graffiti taggers there actually are in a large city. Let's take a hypothetical city of a million people. I would wager there's probably 100-200 active taggers doing most of the graffiti. So as a percentage it's really small. 200 / 1e6 = .0002 or 2 hundreths of a percent.

A small group are responsible for most of the ugly graffiti all around the city.

What percentage of the population has to be "bad" before neighborhoods become "bad" and people start fleeing them?

In the city of Richmond CA, they are targeting the small group of people who are doing the most damage. Not graffiti, but homicides and violent assaults.

http://www.news.com.au/world/north-america/the-town-where-it-literally-pays-to-be-a-criminal/story-fnh81jut-1227328371404

It was this that led Devone Boggan to establish the ONS and it was one key piece of information that caused him to implement his controversial strategy. Upon learning that 70 per cent of the homicides and firearm assaults in 2009 were directly linked to just 17 people, Boggan had an epiphany.

“I thought, ‘Wow, if we can wrap our arms around that and just engage the 17 people in a different way, that could have a significant impact on the narrative of what’s really going on in the city of Richmond.