Part of the study looked at how the query answers were presented to the user. In this case, Google Now showed a separate card with the query answer along with the other search results 58% of the time, while Siri included that knowledge base data 29% of the time, and Cortana just 20% of the time. However, that's really just a matter of formatting and contextual information, a much more important factor is how often your question is answered completely, and in that case Google Now blew away the competition.
Google Now returned the complete answer a whopping 88% of the time, while Siri was at 53%, and Cortana at 40%. It should be noted how that was judged - if the question was "How old is the Great Wall of China?" an answer was only deemed complete if the knowledge card data returned the answer of ~2220 years old. If the card said that the Great Wall was built in 206 BC, that wouldn't count as complete, nor would it count if the search results below contained the answer. Given how much more often Google Now has knowledge cards included, it was already poised to win, but having an 88% success rate for complete answers under those strict guidelines is still very impressive.
We considered giving Cortana a bit of leeway for being the newcomer, but while Cortana is new, the knowledge base it draws on (Bing) has been around since mid-2009. Given Bing's relative age, it's actually a bit sad to see Apple ahead of Cortana, especially since Apple doesn't own a search engine.
Of course, as we noted at the start, this was a competition strictly in terms of getting answers to knowledge base questions, and did not judge differences between virtual assistant functions of each. When it comes to setting reminders, changing phone settings, managing your calendar, sending messages and more, you may have a different take on the strengths of each system.