D.1 Rating Services

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The PICS rating service specifications are designed to enable many different kinds of ratings services on the World Wide Web. A rating service is any person, organization, or other entity that issues ratings. Ratings can be distributed with the document being rated, by a third-party site, on a CD-ROM, or by any other electronic means.

The PICS standard specifies a syntax for text files that describe the different kinds of ratings that a rating service can issue. This lets computer programs automatically parse the kinds of ratings that a service provides.

In their article describing PICS, Resnick and Miller create a sample PICS rating service based on the MPAA's movie-rating scheme:

((PICS-version 1.0)   (rating-system "http://moviescale.org/Ratings/Description/")   (rating-service "http://moviescale.org/v1.0")   (icon "icons/moviescale.gif")   (name "The Movies Rating Service")   (description "A rating service based on the MPAA's movie rating scale")   (category     (transmit-as "r")    (name "Rating")    (label (name "G") (value 0) (icon "icons/G.gif"))    (label (name "PG") (value 1) (icon "icons/PG.gif"))    (label (name "PG-13") (value 2) (icon "icons/PG-13.gif"))    (label (name "R") (value 3) (icon "icons/R.gif"))    (label (name "NC-17") (value 4) (icon "icons/NC-17.gif"))))

This rating description indicates a location where information about the rating system and service can be found, gives it a name, and creates a single rating category called Rating. Rated objects can have one of five different ratings: G, PG, PG-13, R, or NC-17. The standard gives each of these ratings a value and an associated icon to be displayed with the rating.

The PICS rating service description is defined to have a MIME file typeapplication/pics-service . The file is formatted as a list.

The PICS format makes extensive use of name/value pairs. These are formatted as (name value). They are interpreted as "name has the value of value." For example, (min 0.0) means that the particular object being described has a minimum value of 0.0.

The following names are used to describe the ratings services themselves:

PICS-version aVersion: The version number of the PICS standard being supported. Should be 1.1.
rating-system aURL: A URL that indicates the location of a human-readable description of the categories, scales, and intended criteria for assigning ratings.
rating-service aURL: A URL that denotes the location of information used by the rating service itself. This URL is used as the basic URL for all icons and database queries.
icon aString: An icon associated with the particular object that is being described.
name aName: A human-readable name of the object being described.
description aDescription: A human-readable description of the object being described.
category: Introduces a list of elements used to denote a particular category that is supported by this rating service.

If a list begins with the atom category, then the list contains a list of name/value pairs that are used to describe a particular ratings category. The following are supported:

transmit-as aString: The name of the category when it is transmitted in a PICS label.
name aName: The name of the category itself.
min aNumber: The minimum value that a label in this category can have.
max aNumber: The maximum value that a label in this category can have.
multivalue aBoolean: Indicates that an object can have more than one label in the category. Has a value of true or false.
unordered aBoolean: Indicates that the order in which labels are reported has no significance. Can be true or false.
label: Introduces a list of elements that describe a particular label.
Integer: Indicates that the label is transmitted as an integer. By default, PICS ratings are not integers.

Each PICS label is further described by a collection of name/value pairs:

name aValue: The name of the label and its value.

Ratings services can operate label bureaus. A label bureau is "a computer system which supplies, via a computer network, ratings of documents. It may or may not provide the documents themselves."

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