  Aims. Radio observing efficiency can be improved by calibrating and reducing
the observations in total power mode rather than in frequency, beam, or
position-switching modes. Methods. We selected a sample of spectra obtained
from the Institut de Radio-Astronomie Millim\'etrique (IRAM) 30-m telescope and
the Green Bank Telescope (GBT) to test the feasibility of the method. Given
that modern front-end amplifiers for the GBT and direct Local Oscillator
injection for the 30 m telescope provide smooth pass bands that are a few tens
of megahertz in width, the spectra from standard observations can be cleaned
(baseline removal) separately and then co-added directly when the lines are
narrow enough (a few km/s), instead of performing the traditional ON minus OFF
data reduction. This technique works for frequency-switched observations as
well as for position- and beam-switched observations when the ON and OFF data
are saved separately. Results. The method works best when the lines are narrow
enough and not too numerous so that a secure baseline removal can be achieved.
A signal-to-noise ratio improvement of a factor of sqrt(2) is found in most
cases, consistent with theoretical expectations. Conclusions. By keeping the
traditional observing mode, the fallback solution of the standard reduction
technique is still available in cases of suboptimal baseline behavior, sky
instability, or wide lines, and to confirm the line intensities. These
techniques of total-power-mode reduction can be applied to any radio telescope
with stable baselines as long as they record and deliver the ONs and OFFs
separately, as is the case for the GBT.
