quartiles: function(nslots, column, callback, error) {
    var tmpl = _.template('select quartile, max(<%= column %>) as maxamount from (select <%= column %>, ntile(<%= slots %>) over (order by <%= column %>) as quartile from (<%= sql %>) _table_sql where <%= column %> is not null) x group by quartile order by quartile');
    this._sqlQuery(tmpl({
      slots: nslots,
      sql: this.getSQL(),