You cannot convert yearly data into monthly data. I guess that you can model a 49-point time series without any reservation.

I agree with Dr.Atuk,

It is better to use the yearly data for 49 years without any converting. It is the simplest way. A reasonable accuracy could be obtained as well.

You can covert annual data to quarterly or monthly data and vise versa using Eviews . Follow these steps

1. import your annual data on the Eviews or copy from excel to Eviews.

2. In the Eviews workflow, highlight the variables and copy.

3. Right click any where outside the workfile in the Eviews or use Ctr+N to open a new workfile.

4. In the resulting workfile create, specify data frequency by changing from annual to quarterly or monthly etc.

5. Input the start and end date of the data and click on OK.

6. A new untitled workfile will appear with coefficient and residual.

7. Right click on the new workfile and click on paste special.

8. From what results click on specified in source under low to high frequency method and select linear then click on OK all.

9. You now have a new workfile with your data in quarterly or monthly form as you desire.

10. Proceed with your estimation.