Was there any research on correlation between jpeg encoding loss and
perceptual quality of resulting image?
For example, ImageMagic "convert" can recompress existing image with
25-50% loss of quality with no visible degradation of image ( resulting in
smaller image).
So, how much loss can be introduced in JPEG compression without perceived loss
of quality and how much storage space can be saved from it?
Can someone point me to white papars or books that research those questions? 

Thanks,