```ruby
  def ci_plusminus(options = {})
    confidence = options[:confidence] ? options[:confidence] : 0.95
    tag = options[:tag] ? options[:tag] : nil
    require 'statistics2'
    n = (tag ? votes.where(voteable_tag: tag) : votes ).size
    return 0 if n == 0
    z = Statistics2.pnormaldist(1 - (1 - confidence) / 2)
    phat = 1.0 * (tag ? votes_for(tag) : votes_for) / n
    (phat + z * z / (2 * n) - z * Math.sqrt((phat * (1 - phat) + z * z / (4 * n)) / n)) / (1 + z * z / n)
  end
```