Delta Sigma Converters: Resolution
The number of effective bits calculated as follows ENOB = (SNR - 1.76) / 6.02 (this is my reference). So in the delta sigma converters where are able to change to SNR by means of noise shaping, filtering and decimation. 
The plot below taken from Analog devices app note MT-022 show SNR as a function of the number of samples of 1st, 2nd and 3rd, order sigma delta loops.