[R] Difference between summary.lm() and summary.aov()

[John Fox]

Dear Spencer and Alexander,

In this case, physician is apparently a factor with three levels, so 
summary.aov() gives you a sequential ANOVA, equivalent to what you'd get 
from anova(). There no simple relationship between the F-statistic for 
physician, which has 2 df in the numerator, and the two t's. (By the way, I 
doubt whether a sequential ANOVA is what's wanted here.)

Regards,
  John

At 09:17 AM 12/6/2003 -0800, Spencer Graves wrote:
>      The square of a Student's t with "df" degrees of freedom is an F 
> distribution with 1 and "df" degrees of freedom.
>      hope this helps.  spencer graves
>
>Alexander Sirotkin [at Yahoo] wrote:
>
>>I have a simple linear model (fitted with lm()) with 2
>>independant
>>variables : one categorical and one integer.
>>
>>When I run summary.lm() on this model, I get a
>>standard linear
>>regression summary (in which one categorical variable
>>has to be
>>converted into many indicator variables) which looks
>>like :
>>
>>            Estimate Std. Error t value Pr(>|t|)
>>(Intercept)  -3595.3     2767.1  -1.299   0.2005
>>physicianB     802.0     2289.5   0.350   0.7277
>>physicianC    4906.8     2419.8   2.028   0.0485 *
>>severity      7554.4      906.3   8.336 1.12e-10 ***
>>
>>and when I run summary.aov() I get similar ANOVA table
>>:
>>           Df     Sum Sq    Mean Sq F value    Pr(>F)
>>physician    2  294559803  147279901  3.3557   0.04381
>>*
>>severity     1 3049694210 3049694210 69.4864 1.124e-10
>>***
>>Residuals   45 1975007569   43889057
>>
>>What is absolutely unclear to me is how F-value and
>>Pr(>F) for the
>>categorical "physician" variable of the summary.aov()
>>is calculated
>>from the t-value of the summary.lm() table.
>>
>>I looked at the summary.aov() source code but still
>>could not figure
>>it.
>>
>>Thanks a lot.
>>
>>__________________________________
>>
>>New Yahoo! Photos - easier uploading and sharing.
>>
>>______________________________________________
>>R-help at stat.math.ethz.ch mailing list
>>https://www.stat.math.ethz.ch/mailman/listinfo/r-help
>>
>
>______________________________________________
>R-help at stat.math.ethz.ch mailing list
>https://www.stat.math.ethz.ch/mailman/listinfo/r-help

-----------------------------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: jfox at mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox