Sampling system. Simple random sample by means of closed envelopes with a figure inside. We will have 40 envelopes closed, 10 of them will have a circle, 10 a triangle, 10 a star and 10 a heart. They will be put into a box; this will be rotated five times before taking an envelope. Randomly, a person outside the study will take an envelope, will see the chosen figure and will write it down in the register.