Hello,
Thanks for all the help. I have made some more progress in creating the XPATH expression I need and applying it. However, the result, which should comply to the XSD defined in the beginning of the Groovy script, does not validate correctly.
I had the logger print the result of the XPATH, and validated it in XML SPY. The validation was correct.
Can you please take a look at the current version of my script and see if something is wrong? I am also attaching below the complete response and the XPATH result (I had it printed out by the logger).

SOAP RESPONSE:
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<soapenv:Body>
<nsiwspref:GetCompactDataResponse xmlns:nsiwspref="http://localhost:8080/axis/services/NSIService">
<ns1:CompactData xsi:schemaLocation="http://www.SDMX.org/resources/SDMXML/schemas/v2_0/message SDMXMessage.xsd urn:sdmx:org.sdmx.infomodel.keyfamily.KeyFamily=ESTAT:STS:2.2:compact ESTAT_STS_Compact.xsd" xmlns="http://www.SDMX.org/resources/SDMXML/schemas/v2_0/message" xmlns:sts="urn:sdmx:org.sdmx.infomodel.keyfamily.KeyFamily=ESTAT:STS:2.2:compact" xmlns:ns1="http://www.SDMX.org/resources/SDMXML/schemas/v2_0/message">
<ns1:Header>
<ns1:ID>JD014</ns1:ID>
<ns1:Test>true</ns1:Test>
<ns1:Name xml:lang="en">Trans46302</ns1:Name>
<ns1:Prepared>2001-03-11T09:30:47-05:00</ns1:Prepared>
<ns1:Sender id="BIS">
<ns1:Name xml:lang="en">Bank for International Settlements</ns1:Name>
<ns1:Contact>
<ns1:Name xml:lang="en">G.B. Smith</ns1:Name>
<ns1:Department xml:lang="en">Statistics</ns1:Department>
<ns1:Role xml:lang="en">SMith</ns1:Role>
<ns1:Telephone>210 2222222</ns1:Telephone>
<ns1:Fax>210 00010999</ns1:Fax>
<ns1:URI>www.sdmx.org</ns1:URI>
<ns1:Email>lala@sdmx.com</ns1:Email>
</ns1:Contact>
</ns1:Sender>
<ns1:Receiver id="ECB">
<ns1:Name xml:lang="en">European Central Bank</ns1:Name>
<ns1:Contact>
<ns1:Name xml:lang="en">John John</ns1:Name>
<ns1:Department xml:lang="en">Statistics</ns1:Department>
<ns1:Role xml:lang="en">Jack Over</ns1:Role>
<ns1:Telephone>210 1234567</ns1:Telephone>
<ns1:Fax>210 3810999</ns1:Fax>
<ns1:X400>lalala</ns1:X400>
<ns1:URI>www.sdmx.org</ns1:URI>
<ns1:Email>lala@sdmx.com</ns1:Email>
</ns1:Contact>
</ns1:Receiver>
<ns1:DataSetAgency>BIS</ns1:DataSetAgency>
<ns1:DataSetID>BIS_JD_237</ns1:DataSetID>
<ns1:DataSetAction>Append</ns1:DataSetAction>
<ns1:Extracted>2001-03-11T09:30:47-05:00</ns1:Extracted>
<ns1:ReportingBegin>2000-12-01T00:00:00</ns1:ReportingBegin>
<ns1:ReportingEnd>2006-01-01T00:00:00</ns1:ReportingEnd>
</ns1:Header>
<sts:DataSet>
<sts:Series ADJUSTMENT="W" AVAILABILITY="B" COLLECTION="A" FREQ="Q" REF_AREA="IT" STS_ACTIVITY="NS0020" STS_BASE_YEAR="2000" STS_INDICATOR="PROD" STS_INSTITUTION="1" TIME_FORMAT="P1M">
<sts:Obs OBS_CONF="F" OBS_STATUS="A" OBS_VALUE="1.51" TIME_PERIOD="2000"/>
<sts:Obs OBS_CONF="F" OBS_STATUS="A" OBS_VALUE="1.52" TIME_PERIOD="2001"/>
<sts:Obs OBS_CONF="F" OBS_STATUS="A" OBS_VALUE="1.53" TIME_PERIOD="2002"/>
<sts:Obs OBS_CONF="F" OBS_STATUS="A" OBS_VALUE="1.54" TIME_PERIOD="2003"/>
<sts:Obs OBS_CONF="F" OBS_STATUS="A" OBS_VALUE="1.61" TIME_PERIOD="2004"/>
</sts:Series>
<sts:Series ADJUSTMENT="W" AVAILABILITY="B" COLLECTION="A" FREQ="Q" REF_AREA="IT" STS_ACTIVITY="NS0030" STS_BASE_YEAR="2000" STS_INDICATOR="PROD" STS_INSTITUTION="1" TIME_FORMAT="P1M">
<sts:Obs OBS_CONF="F" OBS_STATUS="A" OBS_VALUE="1.51" TIME_PERIOD="2000"/>
<sts:Obs OBS_CONF="C" OBS_STATUS="A" OBS_VALUE="1.52" TIME_PERIOD="2001"/>
<sts:Obs OBS_CONF="C" OBS_STATUS="A" OBS_VALUE="1.53" TIME_PERIOD="2002"/>
<sts:Obs OBS_CONF="C" OBS_STATUS="A" OBS_VALUE="1.54" TIME_PERIOD="2003"/>
<sts:Obs OBS_CONF="F" OBS_STATUS="A" OBS_VALUE="1.61" TIME_PERIOD="2004"/>
</sts:Series>
</sts:DataSet>
</ns1:CompactData>
</nsiwspref:GetCompactDataResponse>
</soapenv:Body>
</soapenv:Envelope>
XPATH RESULT:
<xml-fragment xmlns:nsiwspref="http://localhost:8080/axis/services/NSIService" xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<ns1:CompactData xsi:schemaLocation="http://www.SDMX.org/resources/SDMXML/schemas/v2_0/message SDMXMessage.xsd urn:sdmx:org.sdmx.infomodel.keyfamily.KeyFamily=ESTAT:STS:2.2:compact ESTAT_STS_Compact.xsd" xmlns="http://www.SDMX.org/resources/SDMXML/schemas/v2_0/message" xmlns:sts="urn:sdmx:org.sdmx.infomodel.keyfamily.KeyFamily=ESTAT:STS:2.2:compact" xmlns:ns1="http://www.SDMX.org/resources/SDMXML/schemas/v2_0/message">
<ns1:Header>
<ns1:ID>JD014</ns1:ID>
<ns1:Test>true</ns1:Test>
<ns1:Name xml:lang="en">Trans46302</ns1:Name>
<ns1:Prepared>2001-03-11T09:30:47-05:00</ns1:Prepared>
<ns1:Sender id="BIS">
<ns1:Name xml:lang="en">Bank for International Settlements</ns1:Name>
<ns1:Contact>
<ns1:Name xml:lang="en">G.B. Smith</ns1:Name>
<ns1:Department xml:lang="en">Statistics</ns1:Department>
<ns1:Role xml:lang="en">SMith</ns1:Role>
<ns1:Telephone>210 2222222</ns1:Telephone>
<ns1:Fax>210 00010999</ns1:Fax>
<ns1:URI>www.sdmx.org</ns1:URI>
<ns1:Email>lala@sdmx.com</ns1:Email>
</ns1:Contact>
</ns1:Sender>
<ns1:Receiver id="ECB">
<ns1:Name xml:lang="en">European Central Bank</ns1:Name>
<ns1:Contact>
<ns1:Name xml:lang="en">John John</ns1:Name>
<ns1:Department xml:lang="en">Statistics</ns1:Department>
<ns1:Role xml:lang="en">Jack Over</ns1:Role>
<ns1:Telephone>210 1234567</ns1:Telephone>
<ns1:Fax>210 3810999</ns1:Fax>
<ns1:X400>lalala</ns1:X400>
<ns1:URI>www.sdmx.org</ns1:URI>
<ns1:Email>lala@sdmx.com</ns1:Email>
</ns1:Contact>
</ns1:Receiver>
<ns1:DataSetAgency>BIS</ns1:DataSetAgency>
<ns1:DataSetID>BIS_JD_237</ns1:DataSetID>
<ns1:DataSetAction>Append</ns1:DataSetAction>
<ns1:Extracted>2001-03-11T09:30:47-05:00</ns1:Extracted>
<ns1:ReportingBegin>2000-12-01T00:00:00</ns1:ReportingBegin>
<ns1:ReportingEnd>2006-01-01T00:00:00</ns1:ReportingEnd>
</ns1:Header>
<sts:DataSet>
<sts:Series ADJUSTMENT="W" AVAILABILITY="B" COLLECTION="A" FREQ="Q" REF_AREA="IT" STS_ACTIVITY="NS0020" STS_BASE_YEAR="2000" STS_INDICATOR="PROD" STS_INSTITUTION="1" TIME_FORMAT="P1M">
<sts:Obs OBS_CONF="F" OBS_STATUS="A" OBS_VALUE="1.51" TIME_PERIOD="2000"/>
<sts:Obs OBS_CONF="F" OBS_STATUS="A" OBS_VALUE="1.52" TIME_PERIOD="2001"/>
<sts:Obs OBS_CONF="F" OBS_STATUS="A" OBS_VALUE="1.53" TIME_PERIOD="2002"/>
<sts:Obs OBS_CONF="F" OBS_STATUS="A" OBS_VALUE="1.54" TIME_PERIOD="2003"/>
<sts:Obs OBS_CONF="F" OBS_STATUS="A" OBS_VALUE="1.61" TIME_PERIOD="2004"/>
</sts:Series>
<sts:Series ADJUSTMENT="W" AVAILABILITY="B" COLLECTION="A" FREQ="Q" REF_AREA="IT" STS_ACTIVITY="NS0030" STS_BASE_YEAR="2000" STS_INDICATOR="PROD" STS_INSTITUTION="1" TIME_FORMAT="P1M">
<sts:Obs OBS_CONF="F" OBS_STATUS="A" OBS_VALUE="1.51" TIME_PERIOD="2000"/>
<sts:Obs OBS_CONF="C" OBS_STATUS="A" OBS_VALUE="1.52" TIME_PERIOD="2001"/>
<sts:Obs OBS_CONF="C" OBS_STATUS="A" OBS_VALUE="1.53" TIME_PERIOD="2002"/>
<sts:Obs OBS_CONF="C" OBS_STATUS="A" OBS_VALUE="1.54" TIME_PERIOD="2003"/>
<sts:Obs OBS_CONF="F" OBS_STATUS="A" OBS_VALUE="1.61" TIME_PERIOD="2004"/>
</sts:Series>
</sts:DataSet>
</ns1:CompactData>
</xml-fragment>