There are many easy mistakes to be made in regards to PITCHf/x analysis, as there are very few black and white ways to make certain claims. A pitcher may have a lower velocity away than at home, but that is more likely an issue of the PITCHf/x system at their home park being slightly different than the rest of the parks. The same can be said for movement of pitches.
Understanding that the system is also adapted each year is vital. PITCHf/x is still a relatively young analytical tool, and changes are made each and every year. While a pitcher’s velocity may rise one year to the next, it is important to understand that there could be some coding error that has occurred due to the past version being less good than the current version. This is not a terribly frequent or common issue, but it is something worth noting.
Pitcher release points also often vary. Many pitchers throw certain pitches from slightly different angles or release points than other pitches. This is usually not too big of an issue, as in order to gain a certain type of movement or velocity on a pitch, the release point must be at least slightly altered. What the release points really can be useful for are identifying if there are significant changes made over the course of a season that have caused a certain pitch to become more or less hittable. Analyzing batted ball data, swing and miss rates, or strike percentages in conjunction with an altered release point is recommended instead of simply looking at the release point of a slider and fastball and concluding that a pitch is being tipped.
Being aware that pitch classifications alter is extremely vital. With Brooks Baseball’s new player cards, which have been manually classified, they paint a more accurate picture of which pitches are being thrown. While their classifications are often consistent with the data given to FanGraphs and available at Texas Leaguers, there are situations where the pitch types are different. In analyzing a player’s repertoire, it is important to scan across all platforms to ensure that the pitch frequencies are accurate. What is great about PITCHf/x analysis is that it is very simple to talk to the people in charge of running their websites and they are usually more than happy to discuss with others and educate.
Mike Fast published a great post on this over at The Hardball Times in 2010, in which he elaborated on the common mistakes made when people first jump into PITCHf/x analysis. Dan Brooks also has a great explanation of the Pitch IQ and z-scores used on his player cards at The Hardball Times, which are two very useful and evolutionary ways to evaluate pitches.