The 4 digit PIN number used for ATM processing should always be difficult to guess and not easily associated with the person. In review of 32 million PIN numbers the person’s birthday was the most common PIN setting.

Banking & Credit Cards – Keep your PIN protected
http://securitywatch.pcmag.com/security/294415-pro-tip-for-thieves-the-pin-is-in-the-wallet

QUOTE: So you’ve stolen a wallet. You’ve pocketed the cash, and now you’re standing at an ATM with your victim’s debit card and only a four-digit PIN standing in the way of the bounty. Four-digit numeric PINs aren’t impossible to guess, but at an ATM you only have, what, three guesses before the account is placed on lockdown?  According to some statistical math wizards at Cambridge University, your best bet is to enter the person’s birthday. The researchers modelled 32 million PIN numbers provided by 32 million PIN numbers provided by the RockYou gaming website breach in 2009, iPhone passcodes, and thousands of online surveys.