IIRC last time I saw a study on this it was pretty high. Well in the 95% range. But there would usually be a phone or two on every 737 or larger sized plane not in airplane mode.
That would somewhat shock me to be honest. I make a genuine effort to turn on airplane mode if only because not doing so drains the battery and I bet I miss at least one in 20 times. I would hope such a study was based on measuring ehat pople did as opposed to what they said they did.
