4. [16 marks] An experiment was conducted to investigate the effect of an electric field on wound healing rates in newts (most animals have a weak electric field). Thirty-three (33) newts were randomly allocated between a 'treatment' group and a 'control' group, and on each newt (in both groups) one hind limb was randomly designated the 'treatment' limb. In the treatment group, an electrode was applied to the treatment limb so that the electric field on that limb was changed to 1.5 times its natural value. The other hind limb was not treated. In the control group, no field was applied. Identical incisions were made in both hind limbs of all the newts, and the healing rates of the incisions was measured. The data are in the file NewtHeal2.csv. The columns are; EField: 1 if the newt was in the control group, or 1.5 if the newt was in the treatment group, HealDiff: Difference in healing rate in micrometres per hour (m/h), where Difference = Treatment limb healing rate - Control limb healing rate.

(a) Provide a suitable graphical display that enables comparison of the distributions of healing rate differences for the two values of electric field. [4]

(b) Using a 5% significance level, test the hypothesis that a 50% increase in the natural electric field has no effect on the mean wound healing rate in newts. Take care to state and evaluate assumptions, qualify your conclusions appropriately, and describe any effect you find. [12]