As a general rule, we're best advised to exercise extreme caution when making claims based on one week of baseball. While the games certainly count and playoff odds are already shifting, 25 at-bats, one start or a couple of relief appearances is typically too small of a sample to say anything of substance about a player, never mind something we didn't already know.
Fastball velocity is an exception to this rule; one week's worth of fastball velocity can carry a considerable amount of new information. When we see that Danny Salazar's velocity is down more than 2 mph from last year, or that Tyler Skaggs has added more than 3 mph, we should take notice.
Two important adjustments are needed when evaluating the first week of velocity data. First, we should be aware that pitchers typically lose velocity as they age -- a fact that should have already been cooked into our expectations -- and that pitchers generally don't have their best velocity in April. Combining these two effects, we can anticipate that older pitchers will throw 0.7 mph slower this April than they did last year. Second, we need to account for the fact that some stadium guns are "hotter" than others. Thankfully, Brooks Baseball is on the case, tweaking last year's Pitchf/x park adjustments based on what we've seen over the past week.
Once these adjustments are made, a pitcher's velocity over the first week of the season predicts his velocity over the remainder of the season just as well as his velocity from the entire previous season. The best velocity predictions going forward split the difference, averaging last year's velocity with velocity from the first week of the season, and this holds true for starters as well as relievers.
Most importantly, these changes in velocity have a significant effect on how pitchers perform: Mike Fast -- who now works for the Houston Astros -- showed that every mile per hour lost raises a starter's ERA by an average of a 0.25 runs and a reliever's era by 0.40 runs, and that changes in velocity matter more the harder you throw. Data from the past five seasons bear this out. Pitchers who showed lost velocity in the first week of the season underperformed their preseason Marcel projections, while pitchers with increased velocity outperformed their projections.
By combining this research, we can make the appropriate adjustments to Steamer's preseason pitching projections based on velocities we've seen thus far. Here are the pitchers who have increased their stock most dramatically based on velocity readings from the first week of the season.
Tyler Skaggs, Los Angeles Angels
2013: 90.0 mph fastball
2014: 93.6 mph fastball
Preseason ERA projection: 4.12
ERA adjustment: -0.38
ESPN Insider Keith Law rated Skaggs as the 12th best prospect in all of baseball a year ago. His fastball was expected to range from 91 to 93 mph -- and to touch 94 mph -- with Law noting that "his frame could still handle a little more weight to increase his stamina and maybe add another tick of velocity."