What is `stat_to_sum`? Like the previous comment you'd be much better create a well defined method name instead of passing around magic keywords:

``` ruby
class Activity
  def self.total_distance_between(period)
    between(period).sum(:distance)
  end

  def self.total_ascent_between(period)
    between(period).sum(:ascent)
  end

  def self.total_achievement_between(period)
    between(period).sum(:achievements)
  end
end

class User
  # This method used to be called sum_of_distance_for but this too closely
  # represents the implementation. Total is much more likely to be in general
  # conversation.
  def total_distance_for(challenge)
    activities.total_distance_between(challenge.period)
  end
end
```