Simplex algorithm is said to have exponential worst case time complexity. Yet it is still often used in practice. How can you determine the average time complexity for a certain
The average case complexity is rather difficult to analyze and it depends on the distribution of your linear program. I believe that it was worked out to be polynomial time under some common distributions. I currently cannot find the paper though.
EDIT: Yes, here are the sources:
Nocedal, J. and Wright, S. J. Numerical Optimization. New York: Springer-Verlag, 1999.
Forsgren, A.; Gill, P. E.; and Wright, M. H. "Interior Methods for Nonlinear Optimization." SIAM Rev. 44, 525-597, 2002.
I read it in the first book and apparently it was proven in a separate paper (Forsgren). You could find either in a university library.