Building a Knowledge Base
with VPExpert (VPX.exe)
see also: vpxguide.doc
1
Structure of a rule-based expert system
development software
External
Database
External Program
Expert System
Knowledge Base
Database
Rule: IF-THEN
Fact
Inference Engine
Explanation Facilities
User Interface
User
Developer
Interface
Knowledge Engineer
Expert
2
Sample Problem
A business uses the following temperature control policy in a work
environment.
In spring, summer, autumn and winter during business hours the
thermostat settings are 20, 24, 20 and 18C respectively. In those
seasons out of business hours the settings are 15, 27, 16 and 14C
respectively. The spring months are September, October and
November; summer months are December, January and February;
autumn months are March, April and May, and winter months are
June, July and August. The weekdays are Monday to Friday, and,
Saturday and Sunday are weekend days. Business hours are
between 9 am and 5 pm.
3
Construct a knowledge base
representing this policy.
1. Extract the goals
1. Decide on the facts
1. Construct the rules
4
Structure of a Knowledge Base
Creating an expert system with VP Expert is
basically a question of entering a knowledge
base which consists of three parts:
ACTIONS
RULES
QUERY STAEMENTS
5
Goals and the Actions Block
The FIND statement expresses the goal. The basic form of this
statement is:
FIND variable
This statement activates the inference engine, causing it to consult the
knowledge base of rules until a value is found for the variable.
FIND Thermostat_setting
6
The Actions Block
1. The ACTIONS block consists of statements which control the actions
of the shell.
2. These statements are executed in the order in which they appear.
3. In effect, the ACTIONS block is the `code' which controls the
execution of the inference engine.
7
Example: Actions Block
ACTIONS
DISPLAY "Thermostat Setting~"
FIND Thermostat_setting
DISPLAY "Setting is {#Thermostat_setting}";
Note that it begins with the word ACTIONS and terminates with a
semicolon (no semicolons between statements).
If you want to make sure that the user has time to read a message
before it disappears from the screen, insert a "~" as the last character in
the message.
8
Production Rules
The basic form of a rule is as follows:
RULE rulename
IF antecedent
THEN consequent;
Every rule must have a unique name (up to 40 chars in length) after the
word RULE.
9
Example Rule
RULE Diagnosis_of_measles
IF Diagnosis = measles
THEN Treatment = penicillin;
In this case, Diagnosis and Treatment are variables, and measels and penicillin
are potential values for those variables.
In other words, this rule states that if Diagnosis has the value measels we may
then assign Treatment the value penicillin.
10
Search Algorithm
If the inference engine was currently trying to use this rule to FIND a
value for the variable Treatment, it would:
Check whether Diagnosis had already been assigned a value.
If not, it would use the rules to try to FIND a value for Diagnosis,
returning to this rule once it has done so.
If the value of Diagnosis is measles, then Treatment is assigned the
value penicillin. At that point, the inference engine does no more work n
FINDing a value for Treatment.
If Diagnosis has some other value (including unknown), then the rule
fails, and the inference engine must search for some other rule to give
Treatment a value (if there is none, then Treatment is assigned the value
11
unknown).
Facts
Variables representing basic facts which do not appear as the
consequent of some rule in the knowledge base (that is, are leafs in the
deduction tree), are considered potential questions for the user.
If the inference engine attempts to FIND such a variable, the user will be
prompted for its value.
This is done with ASK and CHOICES statements.
12
Ask Statement
The form of the prompt for a variable is defined by an ASK statement. It
has the form:
ASK variable: "prompt";
For example:
ASK Month : "Which month is it?";
As with any other program, these prompts should be informative -- that
is, they should tell the user how to gather and enter the information.
13
Choices Statement
CHOICES variable: list of values;
For example:
CHOICES Month : july, august;
A menu is printed out when the question is ASKed. Note that if there is
no CHOICES statement for a variable, the user will have to type in a
value at the cursor after the prompt (this is how numeric variables are
usually handled).
14
More rules
Rule 2
IF day = workday
AND Time = between_9am_and_5pm
THEN Operation = business_hours;
Rule 3
IF Month = july
THEN Season = winter;
Rule 4
IF Season = winter
AND Operation = business_hours
THEN Thermostat_setting = 18_degrees;
15
Simple Premises
The IF part of a rule consists of one (or more) simple premises of the form:
variable relational_operator value
where relational_operator is one of the following:
=
<>
<
<=
>
>=
is equal to
is not equal to (used instead of NOT by VP-Expert)
is less than
s less than or equal to
is greater than
is greater than or equal to
The latter four operators are primarily used for numeric values
16
Booleans
RULE
IF
AND
OR
THEN
flu
throat = sore
temperature = high
temperature = very_high
Diagnosis = flu;
is interpreted as (throat = sore) AND (temperature = high OR temperature =
very_high).
17
Variables
VP-Expert does not allow you to declare your variables.
It needs some way to tell a variable (such as diagnosis) from a value (such as
measles) when used on the right side of the expression.
This is done by enclosing the variable on the right side of the expression in
parentheses.
For example:
RULE
IF
THEN
own_pharmacy
pharmacy_used = ours
perscription = (treatment);
18
Multiple answers to queries
The inference engine for VP-Expert will stop once it has found the first value of a
variable it is trying to FIND.
It is possible, however, to force it to find all values for variables instead.
This is done with the PLURAL statement (which must be placed before the
ACTIONS block):
PLURAL: thermostat_setting;
In the a medical knowledge base you wish to make treatment and diagnosis
plural
19
PLURAL inputs
It is also possible to set up your ASK statements to allow the user to choose
more than one option from the menu by making that variable PLURAL.
This is often faster for the user than having to answer multiple questions. For
example:
PLURAL: symptoms;...
ASK symptoms: "Choose all symptoms that you have.";
CHOICES symptoms: sore_throat, spots, rash, none;
The user can then press ENTER by each one which holds, and press END to
confirm the list.
20
!Thermostat Example
ACTIONS
DISPLAY "Thermostat Setting~"
FIND Thermostat_setting
DISPLAY "Setting is {#Thermostat_setting}";
Rule 1
IF Today = monday
THEN day = workday;
Rule 2
IF day = workday
AND Time = between_9am_and_5pm
THEN Operation = business_hours;
Rule 3
IF Month = july
THEN Season = winter;
Rule 4
IF Season = winter
AND Operation = business_hours
THEN Thermostat_setting = 18_degrees;
ASK Month : "Which month is it?";
CHOICES Month : july, august;
ASK Time : "What time is it?";
CHOICES Time : between_9am_and_5pm, before_9am;
ASK Today : "What day is it";
CHOICES Today : monday, tuesday;
21
Explanation and the
BECAUSE statement
RULE measles
IF
temperature = very_high
AND
spots = yesAND innoculated <> yes
THEN Diagnosis = measles
BECAUSE "A high temperature and spots are the usual symptoms of
measles";
The text contained if the BECAUSE statement is printed if this rule is involved in
either a how or why query.
22
Extracting Explanations for a
Consultation
VP-Expert also allows you to see explanations of how a variable was set, or
why a question was asked. This can be done when the program is running
by pressing the / key (which halts execution temporarily) and then selecting
the desired option from the menu.
If how is selected, VP-Expert will then provide a menu of the variables used
in the program. It will then print the BECAUSE part (see below) of the rule
used to give that variable its value (if any).
If why is selected, VP-Expert will print the BECAUSE part (see below) of the
rule responsible for causing the current question to be asked.
Chest congestion and nasal congestion without the sore throat can also
indicate pneumonia.”
IF chest_congestion = yes AND
nasal_congestion = yes
THEN Diseases = pneumonia CNF 85;
BECAUSE Chest and Nasal congestion are indicative of pneumonia
23
Alternate Explanation
It is perhaps more user friendly to print explanations directly using DISPLAY
statements:
RULE measles
IF
temperature = very_high
AND
spots = yes
AND
innoculated <> yes
THEN Diagnosis = measles
DISPLAY "Your extremely high temperature and spots indicate a case of
measles."
24
Confidence Factors
Uncertainty in VP-Expert is represented in terms of ``confidence factors''
(abbreviated CNF), which may take on any value between 0 and 100.
These confidences apply to the assignments done by the inference
engine -- for example, something of the form
diagnosis = flu CNF 80
means that the system believes that diagnosis has the value
measles with certainty of 80 out of 100
RULE flu
IF
temperature > 99
THEN diagnosis = flu CNF 80
25
Certainty in VPExpert
Combining certainties of rules and premises
If both the premise (that is, the ``IF'' part) of a rule and the rule itself are
uncertain, then the confidence of the conclusion of the rule is:
CNF of premise * CNF of rule / 100
VPExpert uses a minimum to compute the confidence of a conjunction and
cf1 + cf2 -(cf1 * cf2)
To compute the confidence of a disjunction
26
Truth Thresholds
This is controlled with the TRUTHTHRESH value in VP-Expert. If the confidence
in the premise of a rule is less than TRUTHTHRESH, then it fails and assigns no
values.
For example, given the rule
IF diagnosis = measles
THEN treatment = penicillin;
If TRUTHTHRESH is 40, and the confidence in the premise diagnosis =
measles is 35, then this premise is considered to be FALSE, and the assignment
treatment = penicillin is not done.
The default value of TRUTHTHRESH is 50. It can be changed in the ACTIONS
block or a rule conclusion. For example, to change the threshold to 40, you
would use the statement:
TRUTHTHRESH = 40
27