  We apply a principal component analysis (PCA) to the spectra of each of the
18 Seyfert 1-like objects observed more than 15 times by the international
ultraviolet explorer (IUE) from 1978 until the end of 1991. PCA allows us to
decompose the Lyman alpha and the C IV (1549 A) emission-lines of each object
into two components with uncorrelated variations. We find that the principal
component describes correlated continuum and line variations, whereas the rest
component reveals the line-parts that do not vary in tune with the continuum. A
cross-correlation analysis in NGC 5548 reveals that variable line features in
the rest component follow the continuum by about 25 days. The symmetry usually
observed in the component's line profiles excludes that the velocity field of
the line-emitting region is dominated by radial motion. In some objects, the
principal component reveals a clearly double-peaked line profile.
  The results suggest that the variable line feature in the principal and the
rest component is respectively emitted in the inner and outer parts of an
extended broad-line region (BLR). The fraction of the line that varies together
with the continuum seems to decrease significantly with increasing luminosity.
We interpret this trend as a consequence of a larger BLR in more luminous
objects. Finally, we propose a solution to the C IV/Ly_a ratio problem without
referring to optically thin emission-line clouds.
