The sample mean arm span (the distance from the middle finger on one hand to the middle finger on the other hand when the arms are extended, measured in cm) for a random sample of 15 U.S. Census at School seventh-grade girls was 155 cm. The sample mean for a random sample of 15 U.S. Census at School eighth-grade girls was 157 cm. Explain why this does not tell us that the mean arm span of all U.S. Census at School seventh-grade girls is different from the mean arm span time of all U.S. Census at School eighth-grade girls.
