iOS 9's Wi-Fi Assist feature uses so much cellular data that Apple faces a multi-million dollar class-action lawsuit over it. Fear not: I've got the details about how to turn it off -- and other advice on holding down your data use.
The lawsuit charges that Apple didn't warn people that the feature would use so much cellular data, and that people have had to pay their phone providers data overcharge fees as a result.
When WiFi Assist is turned on, iOS 9 automatically switches to cellular data when a WiFi signal is too weak.
It's easy to turn the feature off. Open Settings, then select Cellular, scroll to the bottom of the screen, and toggle Wi-Fi Assist to Off. But if you're worried about using too much data, don't stop there. On that same screen, go to the "Use Cellular Data For" section. You'll see all of your apps listed, along with how much cellular data they've been using. Look for any data hogs. Pay particular attention to streaming apps such as Spotify and other video, music, and media apps. If you see that any use too much data, toggle them to Off. From that point on, they'll only use Wi-Fi and not cellular data.