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Home > Forums > Engineering Computer Programs > General Engineering Programs > -Engineering spreadsheets Forum

Exponential trendline fit in Excel using LINEST - fixing a variableHelpful Member! 

thread770-288851
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GrahamSRowe (Bioengineer)
31 Dec 10 3:10
I know that you can use the following formulae in Excel to fit a curve using LINEST for the named ranges x and y:

Exponential Trendline
Equation: y = c *e ^(b * x)
c: =EXP(INDEX(LINEST(LN(y),x),1,2))
b: =INDEX(LINEST(LN(y),x),1)

My question is, if I want to fix c=1 can I simply use

b: =INDEX(LINEST(LN(y),x),1) and assume c=1?

Also would it be correct to use

r2: = INDEX(LINEST(LN(y),x),TRUE,TRUE),3)

in this case?

Thanks very much.
Helpful Member!  Brad1979 (Electrical)
10 Jan 11 15:36
The syntax for LINEST is:

LINEST(known_y's,known_x's,const,stats)

You can force the intercept to be 0 (which is equivalent to c=1) by setting const to FALSE.  So your equations should be:

c: =EXP(INDEX(LINEST(LN(y),x, FALSE),1,2))
b: =INDEX(LINEST(LN(y),x, FALSE),1)

This will cause b to be calculated assuming that c = 1.
Brad1979 (Electrical)
10 Jan 11 15:47
Also,

r2: = INDEX(LINEST(LN(y),x),TRUE,TRUE),3)

this is correct in general but you'll want to change it to the following when you for c=1:

r2: = INDEX(LINEST(LN(y),x),false,TRUE),3)  
Denial (Structural)
10 Jan 11 16:24
Note typo in Brad1979's last two formulae where "x)," should be "x,"
 
Brad1979 (Electrical)
10 Jan 11 16:41
Yes, thank you for catching that.
BigInch (Petroleum)
17 Jan 11 8:33
I always find myself referring to this thread,
http://www.eng-tips.com/viewthread.cfm?qid=214997&page=1
 
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