def record_treatment!(experiment, treatment, object)
      redis.sadd(subjects_key(experiment, treatment), generate_identifier(object))
    end
def record_treatment!(experiment, treatment, object)
      redis.sadd(subjects_key(experiment, treatment), generate_identifier(object))
    end
Data model:

 Key: experiments:<experiment_id>:treatments:<treatment_id>:subjects
 Type: set
 Values: subject identifier

 Key: experiments:<experiment_id>:treatments:<treatment_id>:events:<event_name>
 Type: hash
 Values: subject_identifier => count