I have a few questions about calculating the part worth utilities.

I have analyzed the utility values with the logit approach. What is the mathematical background of the calculation? How exactly are the utilities calculated?

What indicates the size of the ranges of utilities? I have done the same conjoint survey in several cities and partly the results of the importances are identical and so are the relations between the utitilies. However, the size of the ranges of utilities are different (e.g. importances are identical, but in survey 1 e.g. maximum and minimum utility of an attratribute are 0.3 and -0.12, while in survey 2 the maximum and minimum utilities are 0.44 and -0.09). The same pattern or the same factor runs through all utilities of a survey (utility range is by the factor larger in survey 1 than in survey 2). How do these differences occur?

Thank you very much!