The first ever directed SETI search resulted in the rather flimsy conclusion that less than one percent of exoplanets might harbor intelligent alien life.
Based on data from Kepler, SETI (Search for ExtraTerrestrial Intelligence) directed their radio telescopes towards a number of stars believed to have planets similar to Earth, at least in size, orbiting around them.
They looked at radio signals in a narrow spectrum, which, as far as we know, don't happen naturally in the universe. They found nothing in the 86 stars they surveyed.
Researchers picked stars that have planets or planet candidates in the habitable zone, have at least five planets orbiting around them, or have super-Earths with at least a 50-day year.
The telescope was directed at the stars and looked for radio signals in the 1.1 to 1.9 GHz range.
In that range, they looked for signals with a spectrum spread of 5 Hz or less, on the assumption that anything so precise would have been created by intelligent life.
There is no known natural mechanism which would generate a signal like this. Until now, SETI looked at broader stretches of the sky hoping to find something.
While the study didn't find alien life, it did provide an upper limit on the number of alien civilizations which are "noisy" in radio range. Researchers believe that no more than 1% of transiting exoplanet systems harbor intelligent life with activity in this radio range.
Which is to say, since they didn't find anything in the 86 stars they looked at, it must mean that, statistically, these civilizations must be rarer than one in 86.
That's not a lot of data to make too many assumptions. In fact, there are so many factors that you have to take into account when looking at the number that in the end, it doesn't much. It assumes that Kepler data is relevant to the entire galaxy, which in itself is a huge assumption.
It also assumes that alien civilizations would be active and "leaking out" intentionally or not radio signals in this range.
But our own civilization has only been capable of generating powerful enough signals that they would escape into space for less than a century and, with the death of analog TV, it's possible that in a few decades Earth won't be generating radio signals like this.
So what SETI is searching for could be a period of just 100 years, if an alien civilization is 100 years less or more advanced than ours, we would not find it this way.
There's also the resolution of the radio telescope to take into account. All in all, this is all assumptions on top of assumptions, which should give you an idea of just how little we know about our universe.