A paper by Catanzarite et. al. arXiv:1103.1443 says
"Here we show that 1.4% to 2.7% of stars like the Sun
are expected to have earth analog planets....Our result
that Earths are relatively scarce means that a substantial
effort will be needed to identify suitable target stars
prior to these future missions"

A paper by Youdin arXiv:1105.1782 says "...~3 earth like
planets with periods below a year." This is for each
sun-like star. Youdin also writes "...the implications for
habitable earths around Sun-like stars is truly staggering."

Both papers base their conclusions on data from the Kepler
Space telescope.

So who's right? Or are they calculating different things?

Rob