I found time and read Gelman and Hill’s “Data Analysis Using Regression and Multilevel / Hierarchical Models“…Now, please do yourself a favour and get it (of course the paperback version 
). Even for experienced or intermediate (myself) this will be a treat for your eyes and neurons.
Posts tagged with regression
The nls() function has a well documented (and discussed) different behavior compared to the lm()’s. Specifically you can’t just put an indexed column from a data frame as an input or output of the model.
> nls(data[,2] ~ c + expFct(data[,4],beta), data = time.data,
+ start = start.list)
Error in parse(text = x) : unexpected end of input in "~ "
The following will work, when we assign things as vectors.
> nls(y ~ c + expFct(x,beta), data = time.data,start = start.list)
#
# Formula: y ~ c + expFct(x,beta)
#
# Parameters:
# Estimate Std. Error t value Pr(>|t|)
# c 3.7850419 0.0042017 900.83 < 2e-16 ***
# beta 0.0053321 0.0003733 14.28 1.31e-12 ***
# ---
# Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
#
# Residual standard error: 0.01463 on 22 degrees of freedom
#
# Number of iterations to convergence: 1
# Achieved convergence tolerance: 7.415e-06
So many times you’ve in a situation where Excel is your god-sent tool. Yeap, I’m talking about statistics & excel, the 8th sin! I found myself tutoring a couple of days ago and I found myself stuck with Excel. How to perform a stepwise procedure overcoming the burden of VB (which I know nothing about it) or defining cell functions?
There is an add-in that comes handy I must admit, Stepwise Regression Add-In. There is one or two problems when installing and I had a problem with variables names (I solved it using the usual notation x1,x2,…,xn,y I suggest you do the same) but it’s overall good. When in a situation as mine , use it!
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