Ik 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 between years. To make it more specific I will give some examples of my thoughts.

THe main research question will look at pre-adoption (2000-2004) and post-adoption (2005-2008). However I want to compare pre-adoption (2000-2004) with a longer period e.g. (2005-2012). This will provide data about a long-term effect.

My thoughts:
1. I can test pre-adoption (2000-2004) to post-adoption (2008-2012) and compare this to the main hypothesis

2. Pre-adoption (2000-2004) to all post adoption years (2005-2012)(including the years of main hypothesis)

3.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)

4. 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'm not sure if this is possible)

I hope you can see my problem from the information above and recommend what is the best possible approach and test, to test this effect?

Kind regards,