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posted by Colin .
I have 28 participants. Each completed five tests. The maximum score a participant could receive on any one test was 4/4. The lowest score anyone could receive was zero.
I will do a Friedman test for this paired data.
My question: given that I have a very large number of identical ranks (mostly participants getting zero in all tests) can I still go ahead? I just want to know if there is a significant difference in the medians.
Thanks!

Yes, assuming the zero was a valid score, as the person being tested didn't know the subject matter. If the zero is a zero for other reasons (absent, being hardheaded and refusing the test, or similar), then the conclusion on difference is likely to be invalid.
I am reminded of instances were kids put their name on answer sheets, and turn it in as an act of rebellion. You can't make a conclusion about the meaning of scores in that case. 
Thanks very much Bob. I much appreciate your taking the trouble to respond.
All the best!
Colin 
After reading Bob's comment, I can't help but remember the best French student I had purposely getting himself a zero but not only putting a wrong answer but the very worst answer! The principal was so angry he wanted to suspend that student but instead I persuaded him to just delete that score, because I knew why he had done that! I'm happy to say that student went on to be a wonderful surgeon.
Sra (aka Mme)
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