SOURCES
WHAT THE LESSONS LEAN ON

Every number names its source

69 studies, datasets and articles behind the twelve lessons, listed by the lesson that leans on them. Where a number is secondhand, it says so. Where it is my own view, the lesson says so and it is listed at the bottom.

LESSON 1 OF 7 · WHY YOUR GUESS IS PROBABLY WRONG
Arccos via the Wall Street Journal, 18 May 2017 →More than 6 million approach shots, 40% short, 5% long.Also in: Two decisions on every approach
Fogarty and Else 2005 →54 golfers, well calibrated on putting, slightly overconfident on chipping.
Golf Digest and Arccos, 2 February 2023 →1,000 golfers at 7 to 9 handicap, six types, approach loss from 0.22 to 3.65 strokes a round against scratch.
LESSON 2 OF 7 · YOUR HANDICAP IS YOUR GOOD DAY
USGA, 29 April 2025 →The Index "measures your demonstrated ability"; golfers hit their target score about 15-20 percent of the time; 2-5 strokes higher in most rounds; target score is Course Handicap plus par.
Rules of Handicapping, Rule 5 →The Index is the average of the best 8 of the last 20 score differentials.
Shot Scope data via Golf Monthly, updated April 2025 →Birdies, bogeys and doubles a round by handicap. 10 handicap: 7.2 bogeys, 2.88 doubles. 15 handicap: 8.1 bogeys, 4.68 doubles.Also in: Where doubles start
Shot Scope data via Golf Monthly, September 2026 →Greens in regulation, 32% at a 10 handicap and 21% at a 15.Also in: Two decisions on every approach
Arccos data in Golf Digest, May 2026 →2.9 doubles a round for a 10 handicap. A second tracker that agrees.
LESSON 3 OF 7 · HOW SURE CAN YOU BE
Brill and Wyner, June 2025 →553 PGA Tour golfer seasons, 2015 and 2017. Real driving skill in 259, approach in 72, putting in 3. "Putting performance appears nearly indistinguishable from noise."
Golf Digest, Shane Ryan, 16 May 2018 →Broadie on measured rounds and how far any stat can be trusted.
Shot Scope data via MyGolfSpy, 3 April 2025 →Three putts a round by handicap. The 15 handicap figure is 1.9, said as "about twice a round".
LESSON 4 OF 7 · WHERE DOUBLES START
Arccos data in Golf Digest, 14 July 2025 →86% of doubles or worse come from five mistakes. Secondhand.
GOLF.com, November 2019 →Tiger Woods's five scoring rules, which Fawcett teaches.
Golf Digest, 1 May 2024 →Fawcett, "get really good at speed control."Also in: When driver stays in the bag
LESSON 5 OF 7 · TWO DECISIONS ON EVERY APPROACH
Arccos data in Golf Digest, 20 April 2023 →Short misses for golfers who shoot in the 90s, by distance.
LESSON 6 OF 7 · WHEN DRIVER STAYS IN THE BAG
Shot Scope data via MyGolfSpy, 26 November 2025 →Fairways and penalty rate, driver against 3 wood, by handicap. Scratch 48%, 15 handicap 47%. Penalty rate at a 15: 2% driver, 5% 3 wood.
Arccos, 29 October 2025 →Tee shots that end in a penalty or a recovery, 12% scratch, 23% at a 10, 33% at a 20.
Arccos, 8 September 2025 →All golfers, driver 45.5% of fairways against 3 wood 46.4%, median distance 226 against 214.
Arccos, 13 October 2022 →A 3 wood gives up 15 to 20 yards and the loss "isn't worth 2% more of fairways hit."
Lou Stagner's chart in Golf Digest, 17 January 2025 →For about a 10 handicap, 160 yards in the fairway scores the same as 140 in the rough. Secondhand.
Broadie's tables via The DIY Golfer, December 2023 →A golfer who shoots in the 90s, 2.9 strokes from 30 yards in the fairway and 3.4 from 80. Secondhand.
LESSON 7 OF 7 · LOG THE ROUND, DON'T JUDGE IT
Harkin and colleagues 2016 →Monitoring progress improved goal attainment across 138 studies and 19,951 participants. The effect was larger when the record was physical and when it was reported. Not golf.
Arccos data via MyGolfSpy, 15 July 2026 →The study used golfers with at least 5 logged rounds.
OPTIONAL LESSON 1 OF 5 · WINTER PRACTICE AT HOME
Barzyk and Gruber, 2024, Motor learning in golf, a systematic review →The systematic review of motor learning in golf that reports Dail and Christina and the feedback studies.Also in: Before you buy a training aid
Dail and Christina, 2004, Research Quarterly for Exercise and Sport →90 novice golfers; 240 putts in one day against 60 a day for four days. The spaced group was better a day, a week and four weeks later.
Vine, Moore and Wilson, 2011, Frontiers in Psychology →22 elite golfers, mean handicap 2.78. The trained group holed more putts under pressure (60% against 36%) and averaged 1.92 fewer putts a round over 10 competitive rounds. No placebo group.
Uthoff, Sommerfield and Pichardo, 2021, Journal of Strength and Conditioning Research →Systematic review of 20 studies. Combined general and golf specific high velocity training raised clubhead speed about 4.1%. Suggested protocol about 8 weeks.
OPTIONAL LESSON 2 OF 5 · WHICH CLUBS EARN THEIR SPOT
Shot Scope data via MyGolfSpy, 17 February 2026 →Share of 7 iron shots within 5 yards of the golfer's own average. Scratch 18.5%, 5 handicap 17.2%, 15 handicap 14.0%, 25 handicap 12.5%. Sample not stated.
Shot Scope data via MyGolfSpy, August 2025 →Solid strike distance for 4, 5 and 6 iron by handicap. 25 handicap 151, 143, 137. 15 handicap 186, 169, 162.
Golf Digest Hot List 2026, game improvement irons →Pitching wedge lofts run 41 to 43 degrees across 16 sets. PING G440 spec page (42) and Callaway Elyte X spec page (41) agree.
Terry Koehler, GolfWRX, October 2022 →Mid 1990s pitching wedge at 48 degrees. Titleist Vokey gapping page: the 4 to 6 degree rule, a wedge maker's guidance.
Arccos, 3 September 2024 →"golfers with handicaps from 0 to 25 are more likely to hit the green with a hybrid" than the equivalent iron. Sample not stated.
Golf Digest, 28 June 2022 →Arccos measured greens hit with a 7 wood and a 3 hybrid, scratch to 25 handicap, 140 to 220 yards. The 7 wood was higher in 28 of 40 comparisons.
Golf Digest, May 2013 →A fitting claim quoted as a fitter or magazine number, not a study.
GOLF, August 2017 →A fitting claim quoted as a fitter or magazine number, not a study.
Club Champion →A fitting claim quoted as a fitter or magazine number, not a study.
Betzler and colleagues 2012 →A fitting claim quoted as a fitter or magazine number, not a study.
OPTIONAL LESSON 3 OF 5 · BEFORE YOU BUY A TRAINING AID
SuperSpeed →One of the three training aid claims, read from the maker page on 12 October 2026.
Ryp Golf →One of the three training aid claims, read from the maker page on 12 October 2026.
Divot Board →One of the three training aid claims, read from the maker page on 12 October 2026.
Wulf and Su, 2007 →30 novices, 9 iron pitch to a target at 15 metres. The external focus group (the club's motion) scored higher on a next day retention test than the internal focus group (the arms) and the control.
Palmer, Chiviacowsky and Wulf, 2016 →Non golfers putted to a target ringed by a large or small circle. With the circles removed next day, the large circle group was more accurate. Ziv and colleagues, 2019 and Chauvel and colleagues, 2015 point the same way.
MyGolfSpy, 24 December 2024 →Personal launch monitors against a Foresight GC4 indoors and a TrackMan outdoors. More than half had average spin differences above 15% indoors; some above 30%. Carry on the better units within about 1 to 3%.
Golf Insider UK, 24 October 2023 →A budget radar's average face and path errors of about 2 to 4 degrees against a TrackMan 4.
Bishop and colleagues, 2023, Journal of Sports Sciences →13 golfers, two sessions on a TrackMan 4. Carry SEM about 7.8 yards. Spin rate ICC 0.15.
SuperSpeed's own data page →Speed training results reported without a control group.
MyGolfSpy, 13 May 2021 →Speed training results reported without a control group.
Par4Success white paper →Speed training results reported without a control group.
Training aid pricesMaker and media pages read 12 October 2026, rounded to a range.
OPTIONAL LESSON 4 OF 5 · ONE BALL, EVERY ROUND
MyGolfSpy, 14 September 2026 →Robot test; at 85 mph none of the 11 balls below 75 compression beat the Pro V1 control for driver ball speed; the softest was 1.38 mph slower; "there is no swing-speed cutoff"; compression explained about 88% of ball speed change with driver speed, "a straight line, not a cutoff." The control ball is named here, not in the lesson.
MyGolfSpy, 21 August 2026 →Robot, 48 balls. 35 yard wedge: urethane averaged 5,415 rpm, ionomer 3,506. Wet full wedge: urethane kept about 90% of dry spin, ionomer about 55%. Urethane 35 yard spin ran from 4,274 to 5,948 rpm. 7 iron at speed: under 100 rpm between covers (as summarised by a third party; MyGolfSpy's own table has an ionomer ball fourth of 48 for high speed iron spin).
Golf Digest, 2026 Hot List process →Golf Laboratories robot, half wedge shots; non urethane balls "at the high end rolled 15 to 20 feet farther after landing"; no distance advantage either way on well struck shots.
Golfshake, 18 September 2026 →3,000 respondents; 24.4% lose none a round, 50.8% one, 18.3% two, about 6.7% three or more. Self reported.
Today's Golfer →Ball price tiers.
Titleist →A ball fitting tool and its catch.
BallCaddie →A ball fitting tool and its catch.
GOLF.com, 2022 →A ball fitting tool and its catch.
OPTIONAL LESSON 5 OF 5 · TEN MINUTES BEFORE THE FIRST TEE
Fradkin, Finch and Sherman, 2001, British Journal of Sports Medicine →1,040 amateur golfers observed at a private course, a public course and a range over three weeks. 54.3% did any warm-up; of those, 88.7% did air swings on the tee. Observed, not self reported.
Fradkin and colleagues, Pinehurst survey, 2008 book chapter →304 golfers, median age 53, median handicap 13; 18.1% did at least two of aerobic exercise, sport specific stretching and sport specific activity before play. Self reported.
Gergley, 2009, Journal of Strength and Conditioning Research →15 young competitive golfers, crossover; after 20 minutes of passive static stretching, clubhead speed down 4.19%, distance down 5.62%.
Coughlan and colleagues, 2018 →Warm up and clubhead speed, part of the 1 to 5 percent range.
Moran and colleagues, 2009 →Warm up and clubhead speed, part of the 1 to 5 percent range.
Tilley and Macfarlane, 2012 →Warm up and clubhead speed, part of the 1 to 5 percent range.
Langdown and colleagues, 2019 →Warm up and clubhead speed, part of the 1 to 5 percent range.
Mumford and colleagues, 2016, Medicine and Science in Sports and Exercise →Caffeine and golf performance, one of three small studies.
Stevenson, Hayes and Allison, 2009 →Caffeine and golf performance, one of three small studies.
O'Donnell and colleagues, 2024, Sports Medicine →Caffeine and golf performance, one of three small studies.
FROM THE APP'S OWN DATA
PRG app data, 453 rounds from 127 golfersHoles with a penalty went double or worse 72% of the time. Two or more short game shots, more than half. A three putt, almost half. On or near the green with two shots left for par, 4%; otherwise 37%. Pitches from 25 to 50 yards: 19% double or worse when the green was found, 60% when it was missed.Also in: Two decisions on every approach, When driver stays in the bag, Ten minutes before the first tee
FROM ISAAK, HIS OWN ACCOUNT

Not studies. Where a lesson uses one of these, it says so in the text.

2021 NCCAA ChampionshipRounds of 74, 74 and 80, from Cedarville University athletics results. What he made on the 18th hole is his memory, not a record.Also in: When driver stays in the bag
2026 Ohio Mid-AmFinal round, hole 4, par from 50 feet, 4th place. His account.
The 3 and 10 ruleThree rounds to act, ten to trust. His numbers, not a study.Also in: Log the round, don't judge it
His winterA simulator hour once or twice a week, office putting most days.
His bagThe 7 wood swap and the three fitting checks, from about eight years of fitting.
His speed112 to 120 mph in one winter at 18, in a college program, lifting at the same time.
The 3 Ball Questions and Stick With OneHis rule from eight years of being asked which ball to buy.
His tournament routineAbout 45 minutes, more than half of it putting.
WHERE YOUR STROKES GO