Noto Sans is an extended family of fonts based on Open Sans that covers more languages. The design of their glyphs is exactly the same (being Open Sans). Yet the results place Open Sans last at 254 WPM, and Noto Sans 8th at 272 WPM…
- The 'g' character was changed, they are quite different
- For the regular (400) weight, Noto Sans is slightly bolder but also more compact
But you're definitely right, most character shapes look directly inherited from one to the other.
I wonder if the weight & letter spacing change can explain the reading speed differences, or if measurement error / random jitter is actually what explains the different average speed in the study between the two.
"Open Sans is a general use font derived from Noto Sans. Typographers designed Noto Sans for the small screens on Android devices. "
And they mention Noto Sans having hinting for that:
"Also, five fonts have hinting (i.e., are engineered for reading on screens with low resolutions): Arial, Helvetica, Calibri, Noto Sans, and Times."
Unless I'm missing something, that's not quite right. Open Sans, designed by Steve Matteson, was released in 2011. Noto Sans, commissioned by Google to cover a wide array of languages, was first released in 2013. Similarly there is a Noto Serif, based on Droid Serif. Open Sans itself goes back to Droid Sans, an earlier design by Matteson. Anyhow, whichever way it is, the point is that both typefaces share the same design.
But you're right, the typefaces are not exactly the same. Here's a visual comparison: http://www.identifont.com/differences?first=Open%20Sans&seco... As idlewan pointed out above, the lowercase "g" is different, and so is the uppercase "I." There is also a slight weight difference. As you mention, the authors of the paper also note that Noto Sans is hinted, whereas Open Sans presumably is not. So I am wrong in my comment above to call it "the same" typeface. The changes are very subtle though, so it's interesting to see the difference in performance.
Also worth noting is that they tested Arial and Helvetica (http://www.identifont.com/differences?first=Arial&second=Hel...), both hinted and almost identical in weight. Helvetica ranked 4th (283 WPM) and Arial 11th (270 WPM).