Python is a language fundamentally similar to Ruby, that has no static type checking, that's used in similar application domains, etc.... It does not have as strong a culture of unit testing as Ruby, and falls above the line (by a statistically insignificant amount) in figure 5 while Ruby is below (by a statistically significant amount).

It does not appear that the paper took automated testing into account at all. Whether greater use of automated testing in Ruby is responsible for the lower defect rate observed here relative to Python would definitely be an interesting topic for another paper.

Clojure is also dynamically typed by default and below the line. It does not have Ruby's strong unit testing culture, but it does have a design that encourages functional programming, which proponents argue reduces sources of error. It also has a culture of REPL-driven development that arguably fills some of the role of automated testing.