What is 'low' PE ratio?

"You should look for stocks with a low PE ratio." What is 'low'?
Depending on your point of view, low PE ratios could mean:

• PE of 5 or below
• PE of 15 or below
• anything less than the median PE of the S&P 500 industrials
• PE in the bottom 20% of the market
• PE that is less than the annual EPS growth rate of the company (PE/EPSGR ratio less than 1)

All the above precise definitions of 'low' PE have been used by various investors.