We dealt with the computer in computer science by showcasing some hardware in the previous chapter. Now, it's time for the science bit.
It's important to take a scientific approach if you wish to achieve consistently reliable results. Have a methodology or test plan and follow it the same way every time, only changing the thing that you want to measure. Automation can help a lot with this.
It's also important to always measure for your use case on your systems with your data. What worked well for someone else may not work out great for you.
We will talk more about science and statistics later in the book. Taking a simple average can be misleading but it's fine to use it as a gentle introduction. Read Chapter 8, The Downsides of Performance ...