[Indirect methods to estimate hidden population: Second part].

"Hidden populations" are difficult to identify because they have stigmatizing or illegal characteristics. For that reason, determining their size or prevalence in certain contexts is complicated. In those populations, traditional or direct methods, as population surveys, do not usually serve for this purpose, but indirect methods, based on incomplete data sources, can be useful.This work completes the original article published in Revista Española de Salud Pública in 2017: "Indirect methods to estimate hidden populations". Different methods are exposed, showing their indications and bias. To make an estimation as real as possible it is necessary to evaluate carefully the data available and analyze the risk of bias.