How does the Americans for Democratic Action calculate the liberal score? From the website (adaction.org):
ADA's Legislative Committee selects 20 votes it considers the most important during that session. ADA's National Board and/or National Executive Committee approves those votes. Each member recieves 5 points if he/she voted with ADA, and does not receive 5 points if he/she voted against us or was absent. The total possible is 100.