'7-15%' does not look like a lower bound to me.
If there is uncertainty in the lower bound produced by an argument, how else would you write it?
Certainly not
"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]"
Perhaps,
"Analyses of the Kepler results yields a minimum fraction of Sun-like stars with an Earth-sized planet within their habitable zone, of 11 +/- 4% [Petigura et al., 2014]"
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