In a famous long-term study of 400 adults, the subjects' age, income, marital status, health (based on a rating designed by the researchers), and happiness (based on a questionnaire designed by the researchers) were recorded at various times. After analysis of the data, it was found that:

• "Controlling for age, marital status, and health, income did not have a significant effect on the subjects' level of happiness."

Briefly explain what is meant by this statement.