but they go to great lengths to show that there is no difference on either side of this boundary and that the location of the boundary line is completely arbitrary.
Their study pretty much relies on it.

From their conclusion:

"Our identification strategy would be undermined if the Penn patrol boundary was selected because it reflects some natural geographic discontinuity of student living or commercial
properties that the university is particularly interested in protecting. To the best of our knowledge, the patrol zone was selected as part of a negotiation with the University City District,
and there is no evidence that the zone is anything but an attempt to secure the outer ring of the campus and off-campus properties that are considered valuable to the university (Rodin, 2007).

Fundamentally, there is no reason to think that the demographic or risk profile shifts fundamentally at 2 to 3 blocks beyond the boundary in the University City District. A large
number of university students live as far as 50th street, suggesting that street blocks have similar
exposure to student risk groups within our bandwidth. The hiring of additional police services
appears to be an effective approach to reducing crime."