An interventional study was conducted to evaluate the effectiveness of an educational training program in relieving teaching-related stress among primary school teachers. Six hundred eligible teachers with various levels of stress were recruited and allocated to receive the training program (intervention, or treatment) or nothing. Each group has 300 teachers. After one year, 70 teachers reported relief stress in intervention group, while only 20 reported relief in the control group. No loss to follow-up was observed in both groups.

Estimate the risk difference between the two groups and numbers needed to treat. (2 marks)

Explain the meaning of numbers needed to treat in this study. (1 mark)