I read the following in Algorithms 4th Edition by Robert Sedgewick and Kevin Wayne:
Our first qualitative observation about most programs is that there is a problem size that characterizes the difficulty of the computational task. Normaly, the problem size is either the size of the input or the value of a command-line argument. Intuitively, the running time should increase with problem size, but the question is by how much it increases...
Another qualitative observation for many programs is that the running time is relatively insensitive to the input itself; it depends primarily on the problem size. If this relationship does not hold, we need to take steps to better understand and perhaps better control the runnig time's sensitivity to the input. But it does ofter hold, so we now focus on the goal of better quantifying the relationship between problem size and running time
My question is, if the input size (problem size of the program) increases, and thus the running time also increases, why would the running time of the program be relatively insensitive to the input? I'm confused.