There is a manual available on the website of the Amsterdam Content Analysis Lab, written by dr. Knut de Swert. This manual explains step by step how to perform a reliability analysis with Krippendorff’s Alpha in SPSS. You can find the manual here: http://www.polcomm.org/amsterdam-content-analysis-lab/manuals/
First, though, you need to restructure your data in SPSS. You start with an SPSS-file in which all cases are coded by all coders. There is one variable named for instance ‘coder’ and there is an article ID variable. If there are three coders, in your file there will be three cases with the same article ID. You can check whether this is the case, check whether all coders have coded an equal amount of cases, and make sure that there are no missing values in the Article ID, using DATAàDEFINE VARIABLE PROPERTIES or FREQUENCIES. If this is the case you can restructure your data.
First, go to DATA –> RESTRUCTURE and save your dataset when asked.
Choose the second option ‘restructure selected cases into variables’
Put ‘ArticleID’ in the ‘Identifier Variable(s)’ box and ‘Coder’ in the ‘Index Variable(s)’ box.
All the other options are set correctly. If you finish the wizard your dataset will show every variable three times: one time for every coder. You’re data is now ready to be analyzed with Krippendorrff’s Alpha as described by dr. Knut de Swert.