I am conducting a research project where I have a experimental group and control group of companies and a cost data point for a base year and the year after. The years of costs are different for most of the companies so for one company the two costs may be in 2006-2007 while for another they may be from 2001-2002. Also, I do have the costs for other years if necessary.
The base year for each company was chosen because an event occurred during that year and I am trying to see if that event significantly changed a certain cost for that company.
I would like to test to see if the the change in cost is statistically different between the experimental and control group.
Also, I would like to normalize the change in cost in some way, I was thinking of calculating the percent change in cost or dividing each cost by market capitalization but any other suggestions would also be appreciated.
What statistical test/procedure would be best to use in this situation?
Also, considering that the companies can be segmented into various categories like country, market capitalization, etc, would you recommend any other procedures to further analyze the data?
Distributional Assumptions: Company's costs can be considered normal, I checked using a normality test. Sample Size: N=109 for experimental and control group, for a total of 436 data points.