  Nearly 30 years after the first detailed studies of low-ionization structures
(LISs) in planetary nebulae (PNe), we perform a statistical analysis of their
physical, chemical and excitation properties, by collecting published data in
the literature. The analysis was made through the contrast between LISs and
high-ionization structures -- rims or shells -- for a large sample of PNe, in
order to highlight significant differences between these structures. Our
motivation was to find robust results based on the largest sample of LISs
gathered so far. (i) Indeed, LISs have lower electron densities (N$_{e}$[S~II])
than the rims/shells. (ii) The nitrogen electron temperatures (T$_{e}$[N~II])
are similar between the two groups, while a bimodal distribution is observed
for the T$_{e}$ based on [O~III] of the rims/shells, although the high- and
low-ionization structures have T$_{e}$[O~III] of similar median values. (iii)
No significant variations are observed in total abundances of He, N, O, Ne, Ar,
Cl and S between the two groups. (iv) Through the analysis of several
diagnostic diagrams, LISs are separated from rims/shells in terms of
excitation. From two large grids of photoionization and shock models, we show
that there is an important overlap between both mechanisms, particularly when
low-ionization line-ratios are concerned. We found a good tracer of
high-velocity shocks, as well as an indicator of high- and low-velocity shocks
that depends on temperature-sensitive line ratios. In conclusion, both
excitation mechanisms could be present, however shocks cannot be the main
source of excitation for most of the LISs of PNe.
