Four students $P, Q, R$ and $S$ traced the path of a ray of light passing through a glass slab for an angle of incidence $40^{\circ}$ and measured the angle of refraction. The values as measured them were $18^{\circ} ; 22^{\circ} ; 25^{\circ}$ and $30^{\circ}$ respectively. The student who has performed the experiment methodically is