Number distribution of observed dwarfs (red line) and dwarfs with detected radio signal (blue line) with respect to their spectral type. The insert panel shows the detection rate as a function of spectral type, defined by N_r/N, where N_r is the number of dwarfs with detected radio signal in a certain spectral type range; N is the number of observed dwarfs in the same spectral type range. In the spectral type range of M0-T8, the total number of observed dwarfs is 273, while the number of dwarfs with detected radio signal is 34. The zero detection rate around spectral type M2 is most likely due to the limited number of observations. Also shown are the uncertainties of the detection rates, estimated by adopting the binomial distribution at 1σ level for the Gaussian distribution (see Sect. 3).