Before MeF programs are delivered to IRS, contractors perform extensive testing.  Once IRS receives the programs, extensive testing is performed internally to ensure the electronic return data can be received and accurately processed by MeF systems.   MeF programs go through several stages of review before they are used to process live returns.  One of the critical steps is the testing performed by the IRS Product Assurance staff where they insert test data into every line of every form and every instance of supporting data to ensure the data is formatted and displayed accurately by MeF systems.  Product Assurance conducted over 25,000 test cases before MeF was implemented.  Every time an IRS form is changed and affects the XML schema, the entire process is repeated.