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> library(plm)
> ## testing whether parameters in variables work (bug by Ulrich Morawtz)
> data("Produc", package="plm")
> estimation_method <- "within"
> estimation_effect <- "individual"
> zz <- plm(log(gsp)~log(pcap)+log(pc)+log(emp)+unemp,
+           data=Produc, 
+           index=c("state","year"),
+           model=estimation_method, 
+           effect=estimation_effect
+           )
> print(summary(zz))
Oneway (individual) effect Within Model

Call:
plm(formula = log(gsp) ~ log(pcap) + log(pc) + log(emp) + unemp, 
    data = Produc, effect = estimation_effect, model = estimation_method, 
    index = c("state", "year"))

Balanced Panel: n = 48, T = 17, N = 816

Residuals:
     Min.   1st Qu.    Median   3rd Qu.      Max. 
-0.120456 -0.023741 -0.002041  0.018144  0.174718 

Coefficients:
             Estimate  Std. Error t-value  Pr(>|t|)    
log(pcap) -0.02614965  0.02900158 -0.9017    0.3675    
log(pc)    0.29200693  0.02511967 11.6246 < 2.2e-16 ***
log(emp)   0.76815947  0.03009174 25.5273 < 2.2e-16 ***
unemp     -0.00529774  0.00098873 -5.3582 1.114e-07 ***
---
Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

Total Sum of Squares:    18.941
Residual Sum of Squares: 1.1112
R-Squared:      0.94134
Adj. R-Squared: 0.93742
F-statistic: 3064.81 on 4 and 764 DF, p-value: < 2.22e-16
> 
> proc.time()
   user  system elapsed 
   1.12    0.20    1.31