derivedBy in simple and complex types

(Trying another email client to avoid the attachments. Again, profuse
apologies!)

Could someone confirm if the derivedBy attribute is only required when
you derive a complex type, but not when you derive a simple one?

For example, there is the example in the Primer:

<xsd:simpleType name="Sku" base="xsd:string">
 <xsd:pattern value="\d{3}-[A-Z]{2}"/>
</xsd:simpleType>

which validates just fine.

But when I have a complex type definition like:

<xsd:complexType name="populationType" base="xsd:integer">
 <xsd:attribute name="year" use="required">
      <xsd:simpleType base="xsd:integer"/>
 </xsd:attribute>

the XML Schema Validator (XSV) gives me an error and says I must use the
derivedBy extension if the base is present.

thanks,
Liz

Liz Castro
Cookwood Press
http://www.cookwood.com