Guideline for max # of levels in rules?

Question

Hi everyone,
I have two general discussion questions that I'm curious about:
1) Do you use a general guideline for the number of levels you use in your rules? and
2) If so--what are the reasons behind that guideline?
I recall hearing that limiting rules to 3 levels deep is a good practice to follow. However, I don't really know the reasoning behind that.
I find that my rules are easily reaching 5 or 6 levels deep when modelling legislation. And if I were to limit the depth of my rules by making premises conclusions in their separate rules, I'll find my rules to be scattered.
I'm interested to know about your thoughts.
Thanks,
-Isamu

Answer 1

Hi,
1) I think there is no generic guideline on maximum number of levels as such. The number of levels usually depend upon the complexity of rules that you are writing.
2) Although it is recommended to avoid deep nesting of complex rules so that the rules are easier to understand and maintain.
Also, if a rule is simple but has some premises which can be reused in large number of other rules then it is better to separate out those intermediate goals into independent conclusions (and these re-usable rules can be kept in some common rules file).
Example:
RULE 1:
the applicant is eligible for the benefits if
    the applicant is a male and
    the applicant satisfies the salary criteria
        the applicant’s salary > 500 and
        the applicant’s salary <= 5000
RULE 2:
the benefit amount for the applicant = 0.25 * the applicant’s salary if
    the applicant satisfies the salary criteria
RULE 1 can be written as:
RULE 1
the applicant is eligible for the benefits if
    the applicant is a male and
    the applicant satisfies the salary criteria
the applicant satisfies the salary criteria if
    the applicant’s salary > 500 and
    the applicant’s salary <= 5000
As fas as scattering of rules is concerned, i would suggest:
1. Keep all the rules belonging to one section of legislation/policy in one document
For example you can have one rule document per section of the legislation
2. Keep all the rules common across sections in another document
Thanks.