A lawsuit charging Wal-Mart underpays its hourly workers in Missouri has been certified as a class-action suit. The ruling by a circuit judge in Jackson County means the lawsuit represents 160,000 to 200,000 present workers at Wal-Mart, and Sam’s Club stores. The lawsuit was filed four years ago. It accuses Wal-Mart of wage abuse by forcing employees to work off the clock, with refusing to pay them overtime, and with refusing to let them take rest and lunch breaks.