I've given up trying to understand how excel does this.

I just rightclick on that autofill button and drag down. Then I can choose how
I want it to fill.

wrote:

Does anyone know what method eXcel uses to add numbers to the end of a
series. I have specifically been doing work with population history and
projections. When I enter the data for X county from 1870 to 2000 and then
highlight a box and drag it down to expand it fills in the new cells with
projected numbers. I am just wondering what calculations eXcel uses to
generate these projected numbers.

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Dave Peterson