Each year, the early weeks of the season produce some peculiar stats, and that doesn't make it easy to be a Fantasy owner. Justin Masterson already has four wins, a 1.85 ERA and a 1.12 WHIP. Brandon Crawford is hitting .303, and John Buck has seven homers. When April oddities like these arise, the question facing every Fantasy owner is, "how do I respond?"

Every add and every drop has its cost, so it's a good idea for owners to look behind the surface stats to see what is going on under the hood. This is especially true in the first month or two of the season, when statistics are comprised of small samples, but even later in the season, player stats can be deceiving.

Sabermetrics -- the use of advanced statistics in baseball -- has caught on in both the Fantasy and real worlds. The use of metrics that break outcomes into components or measure skill can offer a reality check when a player produces unexpectedly good or bad Fantasy stats. This primer reviews some of the more widely-available peripheral stats that have been developed by sabermetric pioneers, which can shed light on the sustainability of a player's current Fantasy value. (For more information on the derivation and history of the stats covered here plus many others, see the glossary at The Hardball Times.)

For each metric, I'll explain what is being measured, what the major league norm for the stat was last season, and a current example of how the stat can be applied for Fantasy purposes. All season long, I'll be using these metrics in my By the Numbers columns to help you find hidden gems for your rosters as well as drop and sell-high targets.

Note: All current year stats are for games played through Monday, April 22.

Sources: 2012 rates for all major leaguers collectively are from Baseball-Reference.com, except where otherwise noted below.
* Rates are from FanGraphs.com.
** Batted ball rates are from FanGraphs.com, excecpt for infield flyball rate, which is from StatCorner.com.
*** Rates are from StatCorner.com.

Stats for both batters and pitchers

Batted ball rates
What they are: The rates at which balls are hit in a given trajectory. For example, a ground ball rate is equal to the proportion of all hit balls that are grounders.
2012 major league rates**: 21 percent for line drives, 45 percent for ground balls, 34 percent for all flyballs, seven percent for infield flyballs (popups).
Players impacted in 2013: Batted ball profiles matter, because they go a long way in determining whether a hitter has power or if a pitcher is prone to allowing extra-base hits, and they also impact batting averages. Brandon Morrow's cold start can be tied to a 49 percent flyball rate that has helped produce four homers and seven runs. Normally much closer to flyball-neutral, there could be a turnaround in Morrow's future. Colby Rasmus has hit four home runs and four doubles, putting him on a 30-homer and 30-doubles pace, both of which would be firsts for him. He has turned 46 percent of his hit balls into flies, which is not much different than his rates from his previous four seasons. Whether he can sustain that pace depends on the next two measures ...

BABIP
What it is: Batting average on balls in play (excludes home runs and strikeouts). It can be a reflection of a hitter's or pitcher's batted ball profile (e.g., flyball hitter, ground ball pitcher) or a reflection of random fluctuation.
2012 major league rates: .297 overall, .709 on line drives, .238 on ground balls and .131 on flyballs.
Players impacted in 2013: With a 1.69 ERA and 0.53 WHIP so far, Hisashi Iwakuma has looked like an ace, even though he hasn't induced that many strikeouts or ground balls. A .119 BABIP through Monday's games is so far out of line with major league norms and Iwakuma's .283 rate from 2012, so he appears to be due for a serious fall. And as for Rasmus, he has yet to make an out on a line drive (7 for 7), so eventually, some of those potential doubles will turn into outs.

HR/FB
What it is: Home run to flyball ratio. It tends to be fairly steady from year to year, but a huge dip or surge can indicate a change in luck, particularly if the sample of flyballs is small.
2012 major league rate: Eight percent
Players impacted in 2013: Worried about David Price? Don't be. He has only allowed 16 flyballs through his first four starts, but four of them became homers. With more reps, he'll likely get that ratio back near the major league average. The aforementioned John Buck is hitting flyballs at a higher rate than usual, but he is also turning 23 percent of them into homers. With HR/FB ratios of 15 percent or lower in each of his previous three seasons, there is good reason to be skeptical of Buck's power production. The same is true for Rasmus, whose 27 percent ratio is more than double those from his last two seasons.

Stats for batters only

Strikeout rate
What it is: The percentage of at-bats that results in a strikeout.
2012 major league rate: 22 percent
Player impacted in 2013: Nick Hundley is off to a surprisingly good start with a .296 batting average, but it's being boosted by a .417 BABIP that's not built to last. Underneath that high rate on balls in play is a 32 percent strikeout rate that will weigh down his average if it persists.

Walk rate
What it is: The percentage of plate appearances that results in a walk.
2012 major league rate: Eight percent
Player impacted in 2013: Coco Crisp's current 16 percent walk rate is more than double his career mark, which is just under eight percent. Not coincidentally, Crisp has drastically reduced his swing percentage on pitches outside the strike zone. Crisp was more selective earlier in his career, so while he appears due for some regression, he may be rediscovering a skill that has merely been dormant. If the trend continues, Crisp will continue to exceed expectations in the stolen base and runs categories.

Isolated Power (Iso)
What it is: The per-at-bat rate at which a hitter gets extra bases. Also, the difference between slugging percentage and batting average.
2012 major league rate: .151
Player impacted in 2013: A casual glance at Billy Butler's .412 slugging percentage might give the impression that the first baseman's power surge of 2012 was a fluke, but it's the batting average that's weighing that stat down. He is getting extra bases at practically the same rate (.196 Iso) as he did last year (.197 Iso), though a more-than-doubling of his walk rate is cutting into his opportunities to get hits. Also, if Butler were not just 2 for 22 on ground balls, his slash line would look much better. Normally, he would have three more ground ball hits by now, and even if they were all singles, that would give Butler a .275/.422/.471 line, instead of the .216/.375/.412 line he currently sports.

Stats for pitchers only

K/9
What it is: The rate of strikeouts per nine innings.
2012 major league rates*: 7.1 for starters, 8.4 for relievers
Player impacted in 2013: In the early going, Clay Buchholz has shed his more contact-friendly ways, striking out 8.7 batters per nine innings, as compared to his 6.8 career rate. He's not getting hitters to swing and miss more often, but he is getting more called strikes, particularly on his sinker. according to BrooksBaseball.net. He's also getting more vertical movement on that pitch, so it's a trend that owners should keep an eye on. If he can sustain it, Buchholz will be more than just a middle-of-the-rotation type.

BB/9
What it is: The rate of walks per nine innings.
2012 major league rates*: 2.8 for starters, 3.5 for relievers
Player impacted in 2013: Marco Estrada is known for his sharp control, and so far this season, he has compiled a pristine 1.5 BB/9 rate. Fantasy owners might overlook that, as his ERA (4.50) and WHIP (1.33) are both inflated. An 18 percent HR/FB ratio and .338 BABIP have played an important role in bloating those stats, but all of those marks should shrink over the coming weeks. There's nothing wrong with Estrada's control, so owners can use this knowledge to their advantage and target him as a buy-low candidate.

Strikes thrown percentage
What it is: The percentage of all pitches thrown that result in strikes, whether swinging, called, fouled off or put in play.
2012 major league rate: 63 percent
Player impacted in 2013: Ross Detwiler has long shown a knack for efficiency, but with an early 67 percent strikes thrown rate, he's getting through plate appearances even more quickly. That means Detwiler can pile up innings for owners in points leagues. He's also getting more swings and misses, but that hasn't translated into a higher K/9 rate, as he is also allowing a much higher rate of balls in play. By holding opponents to a .167 batting average on grounders, Detwiler has been able to maintain an 0.90 ERA and 1.00 WHIP, but without more strikeouts, those marks could soar to mediocre levels.

Swinging strike percentage
What it is: The percentage of pitches that result in a swing-and-miss.
2012 major league rate***: Nine percent
Player impacted in 2013: Not surprisingly, pitchers who frequently get swinging strikes also tend to get lots of strikeouts. As a rookie, Tommy Milone had a mediocre nine percent rate, and since the lefty doesn't throw hard, owners probably didn't expect much more. However, Milone got swings-and-misses as a minor leaguer, and through four starts, he has lifted that rate to 10 percent, helping himself to a 7.5 K/9 rate. That won't make Milone a stud in the strikeout category, but if he can maintain his gains, he can help with ERA and WHIP without forcing owners to sacrifice Ks.

Strand rate
What it is: The percentage of baserunners who fail to score an earned run.
2012 major league rates*: 72 percent for starters, 75 percent for relievers
Player impacted in 2013: Entering his Tuesday start, Carlos Villanueva was the only qualifying starting pitcher in the majors who had stranded every baserunner he had put on, as all three of the earned runs he allowed came from solo shots. (He finally failed to strand a runner on Tuesday at the Reds.) The home runs should continue to come, and meanwhile, at some point Villanueva will have some men on base when those and other hits happen. Owners in deeper leagues can put this early-season statistical oddity to good use and sell high on Villanueva.

xFIP (Expected Fielding Independent Pitching)
What it is: An estimate of what a pitcher's ERA would be if it were based on outcomes that a pitcher can influence, such as strikeouts, walks and flyballs, factoring out the impact of defense and luck. Derived from FIP, but factors in flyballs in place of home runs.
2012 major league rates*: 4.06 for starters, 3.92 for relievers
Player impacted in 2013: ERA can be deceiving because of the distortions that occur due to aberrant strand or BABIP rates. Hyun-Jin Ryu has been an early example of this. He has been throwing strikes and missing bats, but his 4.01 ERA does not reflect it. xFIP takes Ryu's .364 BABIP into account, building in the assumption that it should be normal, and estimating that his ERA should actually be 2.98. A 28 percent line drive rate has played a role in that inflated BABIP, but as Ryu's innings build up, it's fair to assume that both his line drive and BABIP rates will recede. He is still largely an unknown quantity, but Ryu is showing early signs of being a top Fantasy pitcher.

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