Below is an xpath that works. It returns all the values of the value attribute of the OutputField tag:

//Satellite/multidataset/file[contains(@name, '311multi')]/Data/DriftAnalysis[contains(@name, '311')]/Metric[@name='Detail' and @type='Overlap']/Element[@metrictype='Overlap']/OutputField[@name='Cleared']/@value

But when I try to sum the returned values of the value attribute like the example below I get the error "Expression must evaluate to a node-set error". Can anyone tell me why and how to fix it?

sum(//Satellite/multidataset/file[contains(@name, '311multi')]/Data/DriftAnalysis[contains(@name, '311')]/Metric[@name='Detail' and @type='Overlap']/Element[@metrictype='Overlap']/OutputField[@name='Cleared']/@value)

And here is a snipit of the xml file that I'm zeoring in on.

.
.
.
<Metric name="Detail" type="Overlap">
  <Element name="PFMR Inactive" metrictype="Overlap" category="MAIN" aname="Detail|PFMR Inactive" metricstatistic="UniqueValueOverlap" label="PFMR Inactive">
    <OutputField name="Rome - 20101201" value="2"/>
    <OutputField name="Rome - 20101229" value="2"/>
    <OutputField name="Difference" value="0"/>
    <OutputField name="Cleared" value="0"/>
    <OutputField name="Remaining" value="2"/>
    <OutputField name="New" value="0"/>
  </Element>
  <Element name="FUND-CODE Invalid" metrictype="Overlap" category="MAIN" aname="Detail|FUND-CODE Invalid" metricstatistic="UniqueValueOverlap" label="FUND-CODE Invalid">
    <OutputField name="Rome - 20101201" value="0"/>
    <OutputField name="Rome - 20101229" value="0"/>
    <OutputField name="Difference" value="0"/>
    <OutputField name="Cleared" value="0"/>
    <OutputField name="Remaining" value="0"/>
    <OutputField name="New" value="0"/>
  </Element>
    .
    .
    .
</Metric>
.
.
.

I used the below xml as input

<Metric name="Detail" type="Overlap">
  <Element name="PFMR Inactive" metrictype="Overlap" category="MAIN" aname="Detail|PFMR Inactive" metricstatistic="UniqueValueOverlap" label="PFMR Inactive">
    <OutputField name="Rome - 20101201" value="2"/>
    <OutputField name="Rome - 20101229" value="2"/>
    <OutputField name="Difference" value="0"/>
    <OutputField name="Cleared" value="1"/>
    <OutputField name="Remaining" value="2"/>
    <OutputField name="New" value="0"/>
  </Element>
  <Element name="FUND-CODE Invalid" metrictype="Overlap" category="MAIN" aname="Detail|FUND-CODE Invalid" metricstatistic="UniqueValueOverlap" label="FUND-CODE Invalid">
    <OutputField name="Rome - 20101201" value="0"/>
    <OutputField name="Rome - 20101229" value="0"/>
    <OutputField name="Difference" value="0"/>
    <OutputField name="Cleared" value="0"/>
    <OutputField name="Remaining" value="0"/>
    <OutputField name="New" value="0"/>
  </Element>
</Metric>

used the below template

<xsl:template match="/">

<xsl:value-of select="sum(Metric[@name='Detail' and  @type='Overlap']/Element[@metrictype='Overlap']/OutputField[@name='Cleared']/@value)"/>

</xsl:template>

and got an output as 1. The output is correct. If you still face the error, then please post the xslt code you use and the relevant input xml.