
The low-energy spectra and electromagnetic properties of medium-mass and heavy nuclei are studied within an effective theory written in terms of collective degrees of freedom. The systematic construction of the effective operators, based on the power counting of the theory, allows for the estimation of theoretical uncertainties. The same approach allows for the estimation of uncertainties associated to two-neutrino double beta decay matrix elements, when these are calculated within the single-state dominance approximation. Experimental data is consistently described.