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Read time is a key component of figuring out the right animation sequence. Let's complete the exercise.
Observation: Tooltips can be helpful but are visually intrusive (they break grid layouts and alignment, are bold by design, etc). So they are helpful to draw attention to something, but quickly overstay their welcome if you leave them hanging around for too long.
Let's design the sequence. Presumably what you want to do is:
1. Draw attention to the tooltip
- Here, the opening animation and placement for the tooltip is important. First, make sure the tooltip is placed somewhere the user can see it, and placed close or next to the action you want the user to take. Second, figure out how to use animation, color, or shape to draw the user's attention to it. If you're using animation, I've found a transition of somewhere between 200ms and 600ms is usually enough to rivet attention and avoid cognitive disruption (too fast) or inattention/impatience (too slow).
2. Make sure the user has time to read it.
- You've done this with the Niram test.
3. Make sure the message is effectively worded.
- Can't help you here. Get a good proofreader :-)
4. Dismiss the tooltip before it becomes annoying.
- If you've done a good job with #1 through #3, you should NOT have to keep the tooltip around any longer than a few seconds after it appears. If you are sure you have the user's attention, then you don't even have to wait a few seconds.
That should provide you with a decent framework to drive a design that works for you.