private Period parsePeriod(XMLDocument xmlDoc, String checkDataQueryPath)
     throws XPathExpressionException {
   final String periodValue =
       parseTextValue(xmlDoc, checkDataQueryPath + CHKDATA_DOMAIN_PERIOD_EXPR);
   final String unitValue =
       parseTextValue(xmlDoc, checkDataQueryPath + CHKDATA_DOMAIN_PERIOD_UNIT_EXPR);
   return (periodValue != null)
       ? new Period(PeriodUnit.value(unitValue), Integer.parseInt(periodValue))
       : null;
 }