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Systematic investigation on the coherent population trapping (CPT) effect in the case of single hyperfine (hf) state excitation in Cs is presented. An experimental analysis of the dependence on the laser power density, frequency detuning and polarization, as well as of the effects related to the presence of a buffer gas or of a cell wall coating is made. The CPT effect occurs when a chain of so called Λ level systems is generated by two coherent laser fields coupling two ground state Zeeman sublevels to an excited one. In this configuration CPT can be detected by scanning either the magnetic field or the frequency of one of the two coherent fields. In both cases subnatural width CPT resonances are observed either as a lack of fluorescence (i.e. dark resonances or electromagnetic induced transparency (ElT)) or as an excess of fluorescence (i.e. electromagnetic induced absorption (ElA)).