为什么schema不起作用?

我把以下两个文件放在同一目录下时,  在shiporder.xsd中定义<xs:element name="quantity" type="xs:positiveInteger"/>,可是 当我把文件shiporder.xml中的<quantity>1</quantity>改成 <quantity>gttretretretr</quantity>时不报错,是不是shiporder.xsd没起作用?
请大侠帮我检查一下.
谢谢.
shiporder.xml

<?xml version="1.0" encoding="ISO-8859-1"?><shiporder orderid="889923"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="shiporder.xsd">
<orderperson>John Smith</orderperson>
<shipto>
  <name>Ola Nordmann</name>
  <address>Langgt 23</address>
  <city>4000 Stavanger</city>
  <country>Norway</country>
</shipto>
<item>
  <title>Empire Burlesque</title>
  <note>Special Edition</note>
  <quantity>1</quantity>
  <price>10.90</price>
</item>
<item>
  <title>Hide your heart</title>
  <quantity>1</quantity>
  <price>9.90</price>
</item>
</shiporder>




shiporder.xsd


<?xml version="1.0" encoding="ISO-8859-1" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"><xs:element name="shiporder">
<xs:complexType>
  <xs:sequence>
   <xs:element name="orderperson" type="xs:string"/>
   <xs:element name="shipto">
    <xs:complexType>
     <xs:sequence>
      <xs:element name="name" type="xs:string"/>
      <xs:element name="address" type="xs:string"/>
      <xs:element name="city" type="xs:string"/>
      <xs:element name="country" type="xs:string"/>
     </xs:sequence>
    </xs:complexType>
   </xs:element>
   <xs:element name="item" maxOccurs="unbounded">
    <xs:complexType>
     <xs:sequence>
      <xs:element name="title" type="xs:string"/>
      <xs:element name="note" type="xs:string" minOccurs="0"/>
      <xs:element name="quantity" type="xs:positiveInteger"/>
      <xs:element name="price" type="xs:decimal"/>
     </xs:sequence>
    </xs:complexType>
   </xs:element>
  </xs:sequence>
  <xs:attribute name="orderid" type="xs:string" use="required"/>
</xs:complexType>
</xs:element></xs:schema>

:(