The xPath to get the value of "simple type" variables is /process_data/@variableName.
If you want to combine strings together you need to use the concat function.
ex. concat( /process_data/@variableString1, /process_data/@variableString2,etc).
If you have other data types than strings, you need to convert them to strings using the string() function.
ex. concat(string(/process_data/@variableInt1), string(/process_data/@variableInt2), /process_data/@variableString1)
Jasmin