According to Stanford mathematics and statistics professor Persi Diaconis, the probability a flipped coin that starts out heads up will also land heads up is 0.51. Suppose you want to test this. More specifically, you want to test to determine if the probability that a coin that starts out heads up will also land heads up is more than 0.50. Suppose you flip a coin (that starts out heads up) 100 times and find that it lands heads up 53 of those times.