A crossover study was designed to examine whether using a mobile phone while driving increases the risk of a motor vehicle accident in the Sydney airport tunnel. The study considered 150 drivers with mobile telephones involved in motor vehicle accidents. Each driver's mobile phone call on the day of the accident and during the previous week was analysed using detailed phone billing records. 70 of the 150 subjects had used the mobile telephone during the l0-minute period immediately before the accident, 37 subjects had used the mobile telephone during the same period on the day before the accident, and 13 subjects had used the mobile telephone during both periods.

a) Draw a 2 by 2 table [2 marks]

b) Is there any evidence of a difference in the effect? [6 marks]