@boomerang,
Thanks.
But I have failed to understand you, Boomerang.
Sure the comparison is happended between healthy people and the ill ones. But, for example, though we can compare the body weight between the two groups, we cannot compare the body weight of A group to the height of B group.
And now, in the thread, A = the beneficial effects of physical activity; B = the positive relationships. That is why it causes confusion.