Feldon TerraNova has a posting from one of the researchers on this EQ2 data dump on record. From the report linked here, I find the relevant quotes:
Players were sampled across the game’s servers by inviting all players to participate in a survey if they logged in during the study time window.
After providing consent, players completed an online web-based survey that took about 25 minutes. Players…were promised a special in-game item as compensation. This item, the “Greatstaff of the Sun Serpent” was created by Sony for this unique use. According to the Sony team, the item was made to be desirable for players of all levels because of its rarity and its usefulness in combat for any player, and proved to be a valuable recruiting tool for the survey. A total of 7,129 players (5,719 males and 1,406 females) participated in the survey in just over two days.
Sony Online’s game databases collected data on player actions across four different game types. Players were asked for their age, gender, race, personal income, education, and whether they had children. Relationship measures included questions about relationship quality (“If you are in a relationship, how would you describe the quality of that relationship?”) and co-play of EQ2 (“Do you regularly play EQ2with a romantic partner, e.g. spouse, fiancé, boyfriend/girlfriend?”).