Research assistant wishes to remain anonymous, what to write in the acknowledgement?
There are several reasons for which that person might want to remain anonymous (e.g. s/he has been a victim of stalking). 

I'd then write something like:


  The authors would like to thank an anonymous contributor from Elance for their help in statistical analysis and...


Edit: I amended the sentence above as suggested by Jonathon Wisnoski in a comment ('his help' -> 'their help') to reflect the fact that the gender of the contributor seems unknown to the OP.