On a bright sunny day, to get a good, correct exposure shooting at f16 your iso and shutter speed will be the same.
For example at f1:
iso 100 shutter speed will be 1/100
iso 200 shutter speed will be 1/200
so for same shutter and iso on:
slightly over cast use f11 (shadows are soft around the edges)
overcast use f4 (shadows barely visible)
heavy overcast use f4 (no shadows)