On Nov 12, 2010, at 5:07 PM, David Winsemius wrote:
On Nov 12, 2010, at 4:22 PM, cran.30.miller_2...@spamgourmet.com
wrote:
Hi -
I have a dataframe of (x,y) values. I'd like to fit an exponential
curve to the data for further statistical analysis (pretty much the
same
functionality provided by Excel's LOGEST worksheet array function).
Can
someone point me to the (set of) functions/ package that is best
suited to
provide this functionality? Admittedly, I am a novice in the use of R
statistical functions, so a brief example of how to compute a
correlation
coefficient off a fitted exponential curve would be greatly
appreciated
(though I could probably work through it over time if I knew the
proper R
tools).
Probably (not seeing a clear description of the LOGEST function):
?exp
?log
?lm
?cor
I set up a OO.org Calc spreadsheet which has a lot of Excel work-alike
functions and does have a LOGEST. Giving an argument of x=1:26 and
y=exp(x) to the first two arguments of LOGEST, I get 1 and e. The
OO.org help page says
"FunctionType (optional). If Function_Type = 0, functions in the form
y = m^x will be calculated. Otherwise, y = b*m^x functions will be
calculated."
This might be the equivalent R operation:
> x<-1:26
> y<-exp(x)
> lm(log(y) ~ x)
Call:
lm(formula = log(y) ~ x)
Coefficients:
(Intercept) x
0 1
> exp(coef(lm(log(y) ~ x)))
(Intercept) x
1.000000 2.718282
Note this is not a correlation coefficient but rather an
(exponentiated) regression coefficient.
--
David Winsemius, MD
West Hartford, CT
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