Thanks to Bill James, we come to realize the importance of the strikeout for pitchers, and the stat-minded fans keep close eye on every pitcher's strikeout rate. one of the stats people often like to cite or keep an eye on is K/9, or strikeout per 9 inning. while like OBP a percentage value helps to put things into better perspective. K/9 is a flawed way to look at a pitcher's true strike out ability.
Why? because K/9 measures how many people you strike out per 27 batters retried. but it doesn't take into account something obvious... that during that 27 out few pitchers face the same amount of batters!
It doesn't take a geniues to understand the obvious problem. if a guy struck out 9 while retiring 27 guys but walked 10, gave up 6 runs and 12 hit. that's obviously NOT the same strikeout ability as a pitcher who also struck out 9 . but only allowed 1 walk 8 hit and 3 runs during the process. if no DP or base running outs or men left on were involved. the former pitch faced 55 batters for those 9 strikeouts. while the later faced just 36. the difference is huge. and it's a much better measurement of one's true strikeout ability to use batters faced to corerlate with strikeouts. and shed better light onto the TRUE ability of pitchers to whiff batters.
I really suggest that many of the big stats site start replacing K/9 with K/PA . which is probably a bit more complicated to understand but much more representative of a pitcher's true ability. in the current model. guys who give up more runners actually look better while guys who get people out take hits. particularly those that could induce a lot of a DPs and such. right now if a guy gets knocked around and need to face 60 hitters on average to record 27 out but record 9 Ks while doing so. he has a K/9 of 9 and is considered a strong strike out guy. while a guy who faced only 30 guys but only got 5 K in the process has a K/9 of 5 is considered a low strikeout guy. but in reality the later is probably the better strikeout pitcher.