Because R?s formula interpreter doesn?t understand brackets. Try: Error in terms.formula(formula, data = data) : invalid model formula in ExtractVars Error in terms.formula(formula, data = data) : invalid model formula in ExtractVars Call: lm(formula = speed ~ I(1.5 * dist), data = cars) Coefficients: (Intercept) I(1.5 * dist) 8.2839 0.1104 Hope this helps. jens From: gov2001-l-bounces at lists.fas.harvard.edu [mailto:gov2001-l-bounces at lists.fas.harvard.edu] On Behalf Of Jeremy Hodgen Sent: Sunday, March 09, 2008 5:07 PM To: gov2001-l at lists.fas.harvard.edu Subject: Re: [gov2001-l] Section 5 code why not just use brackets Jeremy On 9 Mar 2008, at 15:43, Jens Hainmueller wrote: Yes you don't necessarily need I() here, but it's a good one to know. It's mostly used in specifying formula objects in model fitting. So for example when you run a linear regression you can use this to compute predictors that are combinations of variables "on the fly" like: lm(Y~I(X1*X2)+I(X3-2/X4),data=Data) For formulas I() simply tells R to take precedence of whatever is within the I() statement. The inside is computed first and then the result is used in the formula. Notice that you can also use formulas for plots like: plot(speed~dist,data=cars) Hope this helps. Jens? From: gov2001-l-bounces at lists.fas.harvard.edu [mailto:gov2001-l-bounces at lists.fas.harvard.edu] On Behalf Of Jeremy Hodgen Sent: Sunday, March 09, 2008 5:54 AM To: gov2001-l at lists.fas.harvard.edu Subject: [gov2001-l] Section 5 code I'm going through the section code & I'm trying to figure out why you've used I() in drawing the first graph - it doesn't seem to make any difference if I take this out. I've checked the help files and it isn't a great deal of help. Jeremy Dr Jeremy Hodgen Senior Lecturer in Mathematics Education King's College London Department of Education and Professional Studies Franklin-Wilkins Building Waterloo Bridge Wing 150 Stamford Street London SE1 9NH Tel: 020 7848 3102 Fax: 020 7848 3182 E-mail: jeremy.hodgen at kcl.ac.uk _______________________________________________ gov2001-l mailing list gov2001-l at lists.fas.harvard.edu http://lists.fas.harvard.edu/mailman/listinfo/gov2001-l Dr Jeremy Hodgen Senior Lecturer in Mathematics Education King's College London Department of Education and Professional Studies Franklin-Wilkins Building Waterloo Bridge Wing 150 Stamford Street London SE1 9NH Tel: 020 7848 3102 Fax: 020 7848 3182 E-mail: jeremy.hodgen at kcl.ac.uk