[Ifeffit] Amplitude and sigma square

Hi all, I have couple of questions. 1. I found in many tutorials that the value of amp should lie between 0.7 and 1. But when I refined it goes to 0.15, what's wrong with it. 2. The value of sigma2. Some time at high k, value, it turns to negative but small. What I suppose to do? 3. If I need to know the behavior of octahedra distortion with temperature, and it is lies within the first shell. So what is the need of higher shell fitting.??? Thanks in advance