Dr. Riddle wants to examine how tax increases influence spen…

Dr. Riddle wants to examine how tax increases influence spending habits. He knows sales tax in Hogsmeade  will increase by 2% at the beginning of 2018. He then has 30 of the biggest retailers report their monthly gross every 3 months for 12 months prior to the tax increase and the every 3 months for the 12 months after the tax increase.  This is an example of what type of research design?