As you scale up, effective means to visualize and manage entire sets of rules at a time becomes more and more important.  Decision tables are excellent in that regard.  In general, decision tables can be used where these three criteria are met:

1. A significant number of rules are parallel -- that is, they share the same subject, have exactly the same evaluation term(s), and are equivalent (but not identical) in effect.  In other words, the rules all share a common pattern and purpose.

2. Each evaluation term has a finite number of relevant instances or ranges of values (a.k.a. brackets).

3. Given the different instances or brackets of the evaluation term(s), the outcomes cannot be predicted by a single formula.  (If a single formula could predict the outcomes, using a single rule or set of rules to give the unified formula is a better approach.)

Here is a simple example.

Rule 1:  Applicable sales tax is to be 6.0% if year = 1999.

Rule 2:  Applicable sales tax is to be 6.5% if year = 2000.

Rule 3:  Applicable sales tax is to be 6.5% if year = 2001.

Rule 4:  Applicable sales tax is to be 6.5% if year = 2002.

Rule 5:  Applicable sales tax is to be 6.25% if year = 2003.

Rule 6:  Applicable sales tax is to be 7.0% if year = 2004.

Rule 7:  Applicable sales tax is to be 8.0% if year = 2005.

Rule 8:  Applicable sales tax is to be 8.15% if year = 2006.

Note that this set of rules satisfies all three criteria above:

1. The eight rules are exactly parallel.  They share the same subject, applicable sales tax; have exactly the same evaluation term, year; and are equivalent (but not identical) in effect, an indicated sales tax percentage for each given year.

2. The evaluation term, year, has a finite number of relevant instances (eight).

3. The outcomes, the percentages indicated for applicable sales tax, cannot be predicted by a formula.  Note that the sales tax rate decreased in 2003 from the previous year -- an apparently infrequent (and perhaps improbable!) tax cut.

The following rule along with Decision Table A shows the consolidated business logic for the eight rules given above.  I think you'll agree it's quite an improvement!

Decision Table A

Year	Applicable Sales Tax
1999	6.0
2000	6.5
2001	6.5
2002	6.5
2003	6.25
2004	7.0
2005	8.0
2006	8.15

The decision table above has only a single evaluation term.  Most decision tables have more than that.  To illustrate, the following rule along with Decision Table B adds a second evaluation term, county, to determine applicable sales tax.

Decision Table B

YEAR	County
	Harkin	Lopes	Qwan	Quail
1999	6.95	8.2	7.35	4.0
2000	6.73	8.3	9.0	4.5
2001	6.15	8.4	9.0	5.0
2002	6.15	8.3	9.0	5.5
2003	6.15	8.4	6.75	6.0
2004	6.15	8.2	6.75	6.75
2005	5.75	8.2	6.75	7.0
2006	5.95	8.4	7.5	7.25

What about representing more than two evaluation terms using a decision table?  That's problematic using a two-dimensional media (for example, paper) unless most of the evaluation terms are simple.  (An example of simple would be is order overdue, which can be answered by only yes or no.)  If the simple test is satisfied, the business logic can still be represented in variations of the two-dimensional format, for example:

• Splitting rows and/or columns within a single array.

• Multiple arrays, with one array per relevant instance or bracket of one (or more) of the evaluation terms.

Beyond that some different approach is required.  At this point (if not before!) an automated support tool becomes just about indispensable.  The following rule along with Decision Table C illustrates.

Decision Table C 

Decision Criteria	Delivery Method for an Order
	Picked Up by Customer	Shipped by Normal Service	Shipped by Premium Service
Rush order	No	Yes	Yes
Order includes fragile item	No	Yes	--
Order includes specialty item	No	No	--
Order includes high-priced item	No	No	--
Order includes item involving hazardous materials	No	Yes	Yes
Category of customer	Silver	Gold	Platinum
Destination of order	--	Local	Remote

Decision Table C establishes the basis for determining the delivery method for an order.  Three possible delivery methods (the outcomes) are indicated along the top.  Seven decision criteria appear at left as labels for the rows.  (This table therefore involves seven dimensions.)  Six of these decision criteria are binary (yes, no or local, remote), whereas one, category of customer, involves three possibilities (silver, gold, platinum).  The choice of delivery method for an order depends on what appears in the cells of a column.  A dash (--) in a cell indicates that the associated decision criteria does not matter in determining the outcome; that is, any alternative for that decision criteria will produce the same outcome.

Such multi-dimension decision tables must be developed with care and then scrutinized closely because they are prone to anomalies and other problems.  For example, consider the issue of completeness.  How complete is the sample decision table above?  Not very!  Can you tell why?  The answer in next month's column....

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