You will sometimes see the statistic k/9 (strikeouts/nine innings) called a pitcher's "strikeout rate". People will talk about this as "how often" a pitcher strikes batters out. The idea is that innings are a measure of opportunities a pitcher has to strike out batters and the strikeouts are how often they are successful. But, in fact, innings measure how many outs a pitcher got whether he had the opportunity to strike out 10 batters or 3. So k/9 actually measures the percentage of outs a pitcher gets that are by strikeout. This makes the idea of it as "strikeout rate" a little odd. We don't, for instance, divide a batter's home runs by their hits to determine their 'home run rate".
Does this matter? I think it does. Because there are two things that effect k/9. It goes up when a pitcher strikes a batter out and it goes down when they induce an out. A high k/9 may be caused by the ability to strike batters out or by the inability to induce batters to make feeble hits. The average ball in play goes for a hit (minus home runs) about 30% of the time - for statheads the major league BABIP is about .300. If you are below average in that regard, as Randy Johnson was, you will need to strike out a higher percentage of the batters you face to be successful. But Johnson is a rarity, most successful pitchers are above average at inducing outs and the best pitchers are usually good at both. If you have a pitcher with a high K/9 you need to look at the hits they are giving up to determine whether that reflects a great strikeout artist or just an inability to get people out any other way.