Japan's labor ministry says an improper method for surveying wages and working hours at private-sector companies was included in a manual used in 2004.
The monthly surveys were supposed to cover all large businesses with 500 or more workers. But the ministry was found to have covered only about one third of such firms in Tokyo since 2004.
The faulty data led to underpayment of unemployment benefits and workers' accident compensation.
According to the ministry, the manual said that even if surveys in Tokyo do not cover all relevant companies, the results will be reliable enough because of the high concentration of those companies in the capital.
The labor ministry also says it failed to take the required procedure of consulting with an internal affairs ministry council when it wanted to change its survey method.