I want to estimate the effect of tax increase on the consumption at the regional level. I have 100 regions in pre-treatment period (year 2010) and the same regions in post-treatment period (year 2011). The tax load was increases on Jan, 1st, 2011. Since, I have a panel for two periods and covariates. I cannot use DID because of the lack of treatment and control groups. If I use a dummy for a year, it will catch other effects and the estimation effect will be biased. What method should be used in this case?