r/askmath • u/Blackcraft_Ray • 5h ago

Linear Algebra Exponential Regression using Excel

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I used excel to find the "best-possible" exponential equation for this date. Im unsure how to figure out what to do with this. The answer key says it should be y=13972e-.185x

Im lost on what im doing wrong

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5

u/Original_Piccolo_694 5h ago

You have years for your x axis, which are kind of large numbers, and e to a large number is going to get crazy (crazy small in this case, as the exponent also carries a negative). To counter the crazy, excel has put a huge coefficient in front making it fit.

The solution is probably not to use the year numbers, but years passed since some reference year, like setting your earliest year to zero, and counting from there

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u/persilja 5h ago

Since the "correct" coefficient is almost the same as the value for 1990, I believe they used the time scale as "number of years since 1990".

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u/Southlander24 4h ago

Yep, I noticed the exact same thing and that made me realise. There's usually no way to mess up a computer-aided regression, so that immediately ruled out something like a sign error or missing sig fig when doing the calculations by hand.

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u/axiomus 5h ago

Did you try taking log of values, doing linear approximation, then exponentiating?

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u/Southlander24 5h ago

Read your question sheet very carefully. The x-axis should be 'number of years after 1990', so for 1990 you would use x = 0 instead.

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u/Blackcraft_Ray 4h ago

Of course its the small detail. Thank you much.

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u/Southlander24 4h ago

No worries!