  Activity classification of galaxies based on long-slit and fiber spectroscopy
can be strongly influenced by aperture effects. Here we investigate how
activity classification for 14 nearby galaxies depends on the proportion of the
host galaxy's light that is included in the aperture. We use both observed
long-slit spectra and simulated elliptical-aperture spectra of different sizes.
The degree of change varies with galaxy morphology and nuclear activity type.
Starlight removal techniques can mitigate but not remove the effect of host
galaxy contamination in the nuclear aperture. Galaxies with extra-nuclear star
formation can show higher [O III] {\lambda}5007/H{\beta} ratios with increasing
aperture, in contrast to the naive expectation that integrated light will only
dilute the nuclear emission lines. We calculate the mean dispersion for the
diagnostic line ratios used in the standard BPT diagrams with respect to the
central aperture of spectral extraction to obtain an estimate of the
uncertainties resulting from aperture effects.
