Exponential trendline fit in Excel using LINEST - fixing a variable 1

[GrahamSRowe]

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,x),1,2))
b: =INDEX(LINEST(LN,x),1)

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

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

Also would it be correct to use

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

in this case?

Thanks very much.

 

[Brad1979]

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

This will cause b to be calculated assuming that c = 1.

 

[Brad1979]

Also,

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

 

[Denial]

Note typo in Brad1979's last two formulae where "x)," should be "x,"

 

[Brad1979]

Yes, thank you for catching that.

 

[BigInch]

I always find myself referring to this thread,

http://www.eng-tips.com/viewthread.cfm?qid=214997&page=1