We predict that ~1.2% of all stars host a planet that may have been capable of supporting complex life at some point in the history of the Galaxy. Of those stars with a habitable planet, ~75% of planets are predicted to be in a tidally locked configuration with their host star. The majority of these planets that may support complex life are found towards the inner Galaxy, distributed within, and significantly above and below, the Galactic midplane.[emphasis mine]
They took into consideration the hazard of supernovae for killing off planetary life, as well as other factors such as the where the necessary heavier elements would be available for producing planets.
You can download the paper here [pdf].