\begin{table}%t7
\caption{\label{tab7}Observed linear polarization, in percentage, from a sample of sources measured at 6 cm at Effelsberg during 2007.}
%\centerline
 {\small \begin{tabular}{llrrrrrrrrrrrrrr}
\hline\hline
 & & Feb. & & Apr. & & May & & Jun. & & Aug. & & Nov. & & Dec. & \\
 Source & & $P$ & $\sigma$ & $P$ & $\sigma$ & $P$ & $\sigma$ & $P$ & $\sigma$ & $P$ & $\sigma$ & $P$ & $\sigma$ & $P$ & $\sigma$ \\
 \hline
0743$-$00$^e$ & & 0.81 & 0.19 & 0.87 & 0.12 & 0.97 & 0.54 & 1.28 & 0.27 & 1.12 & 0.17 & 1.11 & 0.60 & 0.84 & 0.53 \\
0743$-$00$^f$ & & 0.82 & 0.13 & 0.85 & 0.09 & 0.97 & 0.20 & 1.24 & 0.06 & 1.06 & 0.08 & 0.98 & 0.09 & 0.90 & 0.13\\
1253$-$05$^e$ & 3C~279  & 3.25 & 0.28 & 1.24 & 0.21 & 0.81 & 0.45 & 0.96 & 0.40 & 1.14 & 0.36 & 1.42 & 0.86 & 1.11 & 1.63 \\
1253$-$05$^f$ & 3C~279  & 3.15 & 0.08 & 1.17 & 0.03 & 0.82 & 0.03 & 0.91 & 0.02 & 1.11 & 0.04 & 1.16 & 0.04 & 1.06 & 0.00 \\
1519$-$27$^e$ & & -- & -- & 3.62 & 0.71 & 2.96 & 0.25 & 5.79 & 0.58 & -- & -- & 4.70 & 0.62 & -- & -- \\
1519$-$27$^f$ & & -- & -- & 3.50 & 0.01 & 3.20 & 0.16 & 5.83 & 0.01 & -- & -- & 4.55 & 0.27 & -- & -- \\
NGC~6572$^e$ & & -- & -- & 0.09 & 0.79 & -- & -- & -- & -- & -- & -- & 0.21 & 1.27 & 0.31 & 0.77 \\
NGC~6572$^f$ & & -- & -- & 0.16 & 0.01 & -- & -- & -- & -- & -- & -- & 0.24 & 0.27 & 0.48 & 0.59 \\
\hline
\end{tabular}}
\medskip
$^e$~Results obtained with the new calibration procedure; $^f$~results obtained with the traditional calibration procedure.
\end{table}