Students usually find the standard deviation a difficult concept because of its formula. Luckily, understanding its definition is much less important than having a feel for what its numerical value means.
Standard deviation and histogram
From any histogram, you should be able to make a rough guess at the value of the standard deviation.
Similarly, given the mean and standard deviation for a data set, you should be able to draw a rough sketch of a symmetric histogram with that mean and standard deviation.
Exercise capacity of the elderly
In later chapters of CAST, you will meet statistical methods that allow you to properly compare two groups such as these, but we mention here that these methods will be based on the group means and standard deviations.