I am currently writing my Master thesis and in addition to finding a relationship, I examine whether the relationship between X and Y became stronger over time. I use a firm fixed effect model, and this is my base regression:

(1) CFO_TURNOVER = α0+α1INDUSTRY SHOCK+ ΣCONTROL VARIABLES +ε

I have 6 x variables and 6 control variables, the x variables are regressed separately. To examine whether the relationship became stronger, I created a new variable: year*x1 ,, year*x2 etc. (the interaction coefficient)

My question is, can I use year plain and simple use 2006,2007,2008 etc. as a variable., or do I need to create a categorical variable? Also, neither as a control or x-variable, year is included in my base regression. Since every interaction variable should also be in the regression individually, can I just add a variable year to my base regression?

With regards to the interpretation of the coefficient in a regression output, if it is significant, it indicates that the relationship is significantly different over time. A positive significant interaction coefficient would indicate that the effect became stronger over time and a significant negative effect would indicate that the relation became less strong?

Thank you in advance,

Mark