 data (phone numbers that will never work) is a nice-to-have 
feature. 
 
Substitution data can sometimes be very hard to find in large quantities. For example, 
if a million random street addresses are required, then just obtaining the substitution 
data can be a major exercise in itself.   
 
Verdict: Substitution is quite powerful, reasonably fast and preserves the look and feel 
of the data. Finding the required random data to substitute and developing the 
procedures to accomplish the substitution can 