In the paper , we investigate correlation between broad-line and radio variations for broad-line radio galaxy 3C 120 .
By the z-transformed discrete correlation function method and the model-independent flux randomization/random subset selection ( FR/RSS ) Monte Carlo method , we find that the broad H \beta line variations lead the 15 GHz variations .
The FR/RSS method shows that the H \beta line variations lead the radio variations by a factor of \tau _ { ob } = 0.34 \pm 0.01 yr .
This time lag can be used to locate the position of emitting region of radio outbursts in jet , on the order of \sim 5 light-years , from the central engine .
This distance is much larger than the size of broad-line region .
The large separation of the radio outburst emitting region from the broad-line region will observably influence the gamma-ray emission in 3C 120 .