[R] Negative values on output

[Bert Gunter]

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Bert

Bert Gunter
Genentech Nonclinical Biostatistics
(650) 467-7374

"Data is not information. Information is not knowledge. And knowledge
is certainly not wisdom."
Clifford Stoll




On Tue, Aug 19, 2014 at 11:09 AM, sam cruickshank
<sam_l_cruickshank at hotmail.com> wrote:
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> Good afternoon,
> I am completed a linear regression model which I personally am happy with the output and residual charts (although they are a bit bunched).  I have been using visreg() to visualise my data, and although I have logged the dependent variable (as it is cost data), on one of the graphical outputs I am getting negative fitted values for one of the Continents.  Attached is the data used, below is the code for the model and graph.  If anyone has any ideas I would hugely appreciate it, I tried a glm with log link too and this didn't help.
> LM22 <- lm(logTotal ~ logArea + Continent + factor(Method1) + Popdens, data=dummy)
> Output Residuals:     Min       1Q   Median       3Q      Max -1.22742 -0.31675 -0.00909  0.28885  1.20224
> Coefficients:                       Estimate Std. Error t value Pr(>|t|)    (Intercept)            -1.40972    0.77661  -1.815 0.081506 .  logArea                 0.03811    0.04041   0.943 0.354619    ContinentAsia           3.51165    0.80009   4.389 0.000182 ***ContinentAustralasia    3.48549    0.97433   3.577 0.001454 ** ContinentEurope         2.44829    0.82369   2.972 0.006452 ** ContinentGlobal         2.37910    1.00668   2.363 0.026197 *  ContinentNorth America  2.27953    0.62960   3.621 0.001303 ** ContinentSouth America  3.60997    0.77627   4.650 9.22e-05 ***factor(Method1)2        1.11259    0.38386   2.898 0.007696 ** factor(Method1)3        0.82519    0.47470   1.738 0.094459 .  factor(Method1)4       -0.06737    0.67341  -0.100 0.921104    Popdens                -0.14261    0.31675  -0.450 0.656435    ---Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
> Residual standard error: 0.6151 on 25 degrees of freedom  (12 observations deleted due to missingness)Multiple R-squared:  0.6998,      Adjusted R-squared:  0.5677 F-statistic: 5.297 on 11 and 25 DF,  p-value: 0.0002714
> visreg(LM22) gives me a graph with "Africa" being within the realm of negative "Total" which is impossible.  If you tell me how  can provide the data and graphs, but the email was kicked back with them in.
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