Export Frame Rate Correction?

I’ve been trying to export an animation I’ve completed as a movie. I wanted it to play at 15fps so, in exporting the movie I followed these steps to try and get the video to play at the desired 15fps:

-Options—>Movie Options
-Settings---->Motion(Frame Rate:15fps)

However, the video plays at 24fps every time I try and export. Anyone who can help?

This is my first exported animation so apologies if I’m missing something really obvious…