  When analysing HII regions, a possible source of systematic error on
empirically derived physical quantities is the limited size of the slit used
for the observations. A grid of photoionization models was built through the
Aangaba code varying the ionizing radiation spectrum emitted by a stellar
cluster, as well as the gas abundance. The calculated line surface brightness
was then used to simulate slit observations and to derive empirical parameters
using the usual methods described in the literature. Depending on the fraction
of the object covered by the slit, the parameters can be different from those
obtained from observations of the whole object, an effect that is mainly
dependent on the age of the ionizing stellar cluster. The low-ionization
forbidden lines are more sensitive to the size of the area covered by the slit
than the high-ionization forbidden lines or recombination lines. Regarding the
temperature indicator T[OIII], the slit effects are small since this
temperature is derived from [OIII] lines. On the other hand, for the abundance
indicator R23, which depends also on the [OII] line, the slit effect is
slightly higher. Therefore, the systematic error due to slit observations on
the O abundance is low, being usually less than 10%, except for HII regions
powered by stellar clusters with a relative low number of ionizing photons
between 13.6 and 54.4 eV, which create a smaller O++ emitting volume. In this
case, the systematic error on the empirical O abundance deduced from slit
observations is more than 10% when the covered area is less than 50%.
