> m2<-plm(prgdp~sdr1,data=pdata,effect="twoways",model="within")
> summary(m2)#显著
Twoways effects Within Model

Call:
plm(formula = prgdp ~ sdr1, data = pdata, effect = "twoways",
    model = "within")

Balanced Panel: n = 31, T = 13, N = 403

Residuals:
       Min.     1st Qu.      Median     3rd Qu.        Max.
-0.25019965 -0.04345968  0.00094265  0.04009926  0.25540659

Coefficients:
     Estimate Std. Error t-value Pr(>|t|)   
sdr1 0.080460   0.026147  3.0772  0.00225 **
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Total Sum of Squares:    2.2224
Residual Sum of Squares: 2.1653
R-Squared:      0.025698
Adj. R-Squared: -0.091001
F-statistic: 9.46908 on 1 and 359 DF, p-value: 0.0022502