Also, seeing stuff like this really bugs me:
top of page 2: "Recent analyses of the Kepler statistics showed that about 20% of all Sun-like stars have Earth-sized planets orbiting within the habitable zone [Petigura, Howard and Marcy 2014]."
2nd paragraph of page 3: "Analyses of the Kepler results shows that 7-15% of the Sun-like stars have an Earth-sized planet within their habitable zone [Petigura et al., 2014]"
That's a pretty glaring error to be making. This isn't a top journal, but it isn't an obscure one either. http://eigenfactor.com/rankings.php?bsearch=International+Journal+of+Astrobiology&searchby=journal&orderby=eigenfactor
I agree. However, considering that Kepler is not actually sensitive enough to detect Earth sized planets in the habitable zone of sun-like stars, both these numbers are extrapolations and it must be assumed that the 7-15% or 20% are well within each other's error bounds.
On the abundance of extraterrestrial life after the Kepler mission Amri Wandel
Some recent calculation of the Drake Equation with estimates of the likelihood and logevitiy of civilizations
Related: The Great Filter and Planets in the habitable zone, the Drake Equation, and the Great Filter