User input data validation should occur on client side.
Then for each transaction there is no need to contact with 
server thereby  transactions decreases and efficiency 
increases.

user input data validation can occur both side means client 
side as well as server side. but generally validated on 
client side because in this validation no any sever 
resources used for validation. So it provides  less 
overhead on server and application performance become fast 
rather than server side. if we validate user input data on 
server side every time data check on server so the 
performance of application become slow down.