Time Series-Regression analysis- Monthly Revenue instead of Quarterly revenue I am studying Time series with Regression analysis.
I have data that has seasonality and trend. I am using Dummy variables for seasonality- In this case I am using quarters.
I selected Quarter 1, Quarter 2 and Quarter 3 and obtained the following model:
$ Revenue= \beta_0+\beta_1{X_{1t}}+\beta_2({X_{S1,t}})+\beta_3({X_{S2,t}}) + \beta_4({X_{S3,t1t}})$ 
Or equivalently: 
$ Revenue= \beta_0+\beta_1{X_{1t}}+\beta_2(Qrt1)+\beta_3(Qrt2) + \beta_4(Qrt3)$ 
However if instead of having the quarterly revenue I have available the monthly revenue how should I analyse this time series?
I am confused regarding this question. I do not understand what is suppose to do, if I have the monthly revenue, I think instead of:
$ revenue= \beta_0+\beta_1{X_{1t}}+\beta_2({X_{S1,t}})+\beta_3({X_{S2,t}}) + \beta_4({X_{S3,t1t}})$, 
I should have
$ revenue=\beta_0+\beta_1(Jan)+\beta_2(Feb)+\beta_3(March)+\beta_4(April)+\beta_5(May)+\beta_6(JUn)+\beta_7(July)+\beta_8(Aug)+\beta_9(Setp)+\beta_10(Oct)+\beta_11(Nov)$ 
But I am not really sure what is meant by this question.
Can anyone help me on this?
Thanks
I edited the question to add the following information:
I have the following data:

The question is: If I have available Monthly revenue, how should I analise the time series? 
 A: If you have monthly revenue data and the goal is to determine the regression coefficients for quarterly revenue, then you would have to categorize each month into the appropriate quarter. For example - if the 1st quarter is defined as October/November/December, then the revenue for these months should be grouped into the variable Qrt1. However, not all companies define their quarters/seasons the same...so make sure you're defining them correctly.
In this data set there is no monthly data. Based on this table, the quarterly data cannot be converted into monthly data. If you have monthly data, then you can easily convert it into quarterly/seasonal data by creating a new categorical variable and then assigning the months accordingly.
If you're trying to compare two different data sets, then they need to be in the same format. In other words, you can't compare regression coefficients for quarterly data to regression coefficients for monthly data. As for the cubic spline...again, it depends on the question. There isn't much data here to work with so I wouldn't recommend it (rule of thumb is 10 per variable but again, it seems like there's info missing in the question).
I hope this answers your question. If my answer is unclear or isn't what you need, let me know.
