[R] [NB] lm problems

[Thomas Lumley]

On Tue, 26 Nov 2002, Matej Cepl wrote:

> Hi,
>
> I have probably overlooked something obvious, but could anybody
> help me with following, please?
>
> Trying to make regression analysis. I have a huge dataframe with
> results from National Opinion Survey on Crime and Justice
> (www.abacon.com/fox/) with two variables G5 and N3 which are
> imported to R as ordered factors:
>
> 	> levels(noscj$G5)
> 	[1] "Strongly agree"    "Agree"             "Neither"
> 	[4] "Disagree"          "Strongly disagree"
> 	> levels(noscj$N3)
> 	[1] "Serious problem"  "Somewhat problem" "Minor problem"  "Not
> 	a problem"
> 	>
>
> (missing values are duly recoded as NA). When I try linear
> regression I get a lot of warnings which I have not managed to
> parse succesfully:
>
> 	> lm(G5 ~ N3,data=noscj)
>
> 	Call:
> 	lm(formula = G5 ~ N3, data = noscj)
>
> 	Coefficients:
> 	(Intercept)         N3.L         N3.Q         N3.C
> 	    3.38087     -0.05821     -0.15364      0.04367
>
> 	Warning message:
> 	"-" not meaningful for ordered factors in: Ops.ordered(y,
> 	z$residuals)
> 	> summary(lm(G5 ~ N3,data=noscj))
>
> 	Call:
> 	lm(formula = G5 ~ N3, data = noscj)
>
> 	Residuals:
> 	[1] <NA> <NA> <NA> <NA> <NA>
> 	Levels:  Strongly agree Agree Neither Disagree Strongly disagree
>
> 	Coefficients:
> 	            Estimate Std. Error t value Pr(>|t|)
> 	(Intercept)  3.38087
> 	N3.L        -0.05821
> 	N3.Q        -0.15364
> 	N3.C         0.04367
>
> 	Residual standard error: NA on 980 degrees of freedom
> 	Multiple R-Squared:    NA,      Adjusted R-squared:    NA
> 	F-statistic:    NA on 3 and 980 DF,  p-value: NA
>
> 	Warning messages:
> 	1: "-" not meaningful for ordered factors in: Ops.ordered(y,
> 	z$residuals)
> 	2: "^" not meaningful for ordered factors in: Ops.ordered(r, 2)
> 	3: ">" not meaningful for factors in: Ops.factor(qs[i], -Inf)
> 	4: "+" not meaningful for factors in: Ops.factor(qs[i],
> 	.minus(x[hi[i]], x[lo[i]]) * (index[i] - lo[i]))
> 	>
>
> Could anybody tell me, what's going on, please? I have no clue
> what "^", ">", etc. means.
>

Well, all the warnings are of the form "  not meaningful for ordered
factors".  The problem is that lm is not meaningful for ordered factors.

You appear to want a linear regression model where the response is
1,2,3,4,5 according to the levels of G5. You need to define a variable
like that.  You can probably just use

numG5<-unclass(noscj$G5)


	-thomas

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