Rachel Garnshaw

Im am getting the following issue when I try validate my XML against the schema. This occurs because I use the same filed in a number of different section. The field is defined once at the top of the schema
<xs:element name="Tin_cost" type="xs:string"/>
It is used 4 times in the schema though as follows
<xs:element ref="Tin_cost" minOccurs="0"/>

Could anyone help me with this issue. How could I resolve this?
Thanks in advance