I have a problem in determining the right statistical test for my research. First I will give some background information. I want to research the impact of IFRS (regulatory change) on audit fees (fees payed to external auditors). This change was mandatory in the year 2005. I will use several pre-adoption years (2000-2004) and several post-adoption years (2005-2008), so I think I can use an OLS regression for this.
However now there is a problem. I also want to test the long term effect of IFRS to see if there is a difference in the years (2008-2012). To make it more specific I will give some examples of my thoughts.
E.g. I can test pre-adoption (2000-2004) to post-adoption (2008-2012)
Pre-adoption (2000-2004) to all post adoption years (2005-2012) (include the years of main hypothesis)
Pre-adoption (2000-2004) to change in audit fees per year, so not necessarily a median change. (this one doesn't lose statistical power over time if I'm correct)
Or use a logit model to see if there is a change yes=1 or no change=0 between the years (2004-2008) and (2009-2012).
I hope you can see my problem from the information above and recommend what is the best possible test to test this effect?