Only certain religious group publish updated figures on their official membership, and this kind of data is collected in order to levy taxes on the registered membership of those churches, which corresponds to 9% of the total income tax (8% in Baden-Württemberg). Many members choose not to formally register to their church anymore in order to have lighter taxes, since that those who opt out don't have the duty to pay it. According to a study, approximately 44% of the persons who unregistered to their church in 2018 did so in order to avoid to pay the church tax. According to a 2017 study by the Pew Research Center, around 20% of people who are not registered to any church consider themselves Christians. Therefore, the official church count may underestimate the actual number of people who consider themselves as Catholic or Protestant, as noted by the 2011 Census, which provides comparable data on the religious statistics based on self-identification and the church register.