Yet another forecasting question.....

If given dates and amount of items received you can easily plot the data and
have Excel produce a fitting formula that's linear, polynomial, logorithmic,
etc.

I am working with different types of data, some linear (which is easy) some
cyclical. For the cyclical data I would like to use either a 3rd or 4th
degree polynomial to approximate the upcoming volume. Again, Excel will plot
the points and give you a formula, but you can't use it to forecast anything.

I thought of using a linear algebra approach and using this formula:

a1=INDEX(LINEST(C3:C10,B3:B10^{1,2,3,4,5}),1)
a2=INDEX(LINEST(C3:C10,B3:B10^{1,2,3,4,5}),1,2)
a3=INDEX(LINEST(C3:C10,B3:B10^{1,2,3,4,5}),1,3)
a4=INDEX(LINEST(C3:C10,B3:B10^{1,2,3,4,5}),1,4)
a5=INDEX(LINEST(C3:C10,B3:B10^{1,2,3,4,5}),1,5)
a6=INDEX(LINEST(C3:C10,B3:B10^{1,2,3,4,5}),1,6)

Where column C houses volumes and B houses dates. I thought that you could
then apply this

f(x) = (a1*x^5) + (a2*x^4) + (a3*x^3) + (a4*x^2) + (a5*x) + a6

where X is the date you are projecting volume for.

Using MS Excel XP if it matters. Any ideas?

Thanks in advance - Kris.