new to schema: Unique Particle Attribution

Hi all,

I am new to xml schema and to this mailing list (if misuse it please 
inform me).

I would like to write an element (question) which does the following:

question can have either:

title(optional), content and weight

or:

title, content(optional), weight(optional) and question

I tried to implement the above rules in an xml schema but I got a 
"Unique Particle Attribution". Below I provide you with the xml fragment 
that implement the rules. Can you tell me how I can implement these rules?

    <xs:group name="simpleQuestionItems">
        <xs:sequence>
            <xs:element ref="title" minOccurs="0" maxOccurs="1"/>
            <xs:element ref="content"/>
            <xs:element ref="weight"/>
        </xs:sequence>
    </xs:group>

    <xs:group name="complexQuestionItems">
        <xs:sequence>
            <xs:element ref="title"/>
            <xs:element ref="content" minOccurs="0" maxOccurs="1"/>
            <xs:element ref="weight" minOccurs="0" maxOccurs="1"/>
            <xs:element ref="question" minOccurs="1" 
maxOccurs="unbounded"/>
        </xs:sequence>
    </xs:group>

    <xs:element name="question">
        <xs:complexType>
            <xs:choice>
                <xs:group ref="simpleQuestionItems"/>
                <xs:group ref="complexQuestionItems"/>
            </xs:choice>
            <xs:attribute name="required" type="xs:boolean" 
default="false"/>
        </xs:complexType>
    </xs:element>