Two Sessions, Six Months Apart

Comparing two sets of fitting numbers taken on different days is one of the most common things a golfer wants to do and one of the easiest things to get wrong. A launch monitor prints a carry figure to one decimal place, which makes it look like a fact. It is a fact about a specific ball, hit with a specific club, in specific air, measured by a specific instrument, by a golfer in a specific state of warm-up. Change any of those and the number moves without the golfer having changed at all.

Here is the checklist we work through before we let two sessions be compared.

1. Was It the Same Ball?

This is first because it is the largest single source of disagreement and the easiest to overlook. A range ball and a premium urethane ball can differ by ten yards of driver carry and far more in wedge spin, and the gap is not a fixed percentage — it grows as loft increases.

If the ball changed between sessions, stop. Nothing else on the sheet is comparable until you account for it, and the honest correction is to re-test rather than to apply a fudge factor.

2. Was It the Same Instrument Type?

A camera unit indoors reports a modelled carry. A radar unit outdoors reports a measured one. Those are different quantities that happen to share a name. Comparing them without noting the difference is like comparing a weather forecast to a thermometer reading.

More on that distinction →

3. Was the Unit Normalising?

Most launch monitors offer a mode that adjusts output to standard conditions — typically sea level, around 21°C, dry. It is a genuinely useful setting because it makes sessions comparable across dates. It is also frequently left on without the golfer knowing, which means the printed number is not what happened in the room.

Two sessions are comparable if both were normalised or neither was. One of each is the worst case, and it is common.

4. What Was the Air Doing?

If neither session was normalised, air density matters. Warm air is less dense than cold air, and — contrary to almost everyone's intuition — humid air is less dense than dry air, because water molecules are lighter than the nitrogen and oxygen they displace. Less dense air means less drag and a little more carry.

On the coast in August, a driver can carry meaningfully further than the same driver in January, with no change to the swing whatsoever. We log temperature and relative humidity at the top of every outdoor block for exactly this reason.

The coastal air question in detail →

5. Was the Golfer Warm?

Speed on shot five is not speed on shot twenty-five, and speed on shot ninety is not either of those. A comparison drawn between an early block in one session and a late block in another is measuring warm-up, not equipment.

Our sheets record shot numbers within the session so this is visible. It is a boring detail that resolves a surprising number of arguments.

6. Same Shot Count, Same Trimming?

An average across eight shots and an average across thirty shots are not equally trustworthy, and an untrimmed average is dominated by whichever block happened to contain a flush strike. We drop the two best and two worst from every block before averaging, and we say so on the sheet, because a trimmed mean compared to a raw mean is another false difference.

The Short Version

1. Same ball, or the comparison is void.
2. Same instrument category, or label which is modelled.
3. Normalisation on for both or off for both.
4. Log air temperature and humidity if not normalised.
5. Compare like-for-like points in the session, not early against late.
6. Same trimming rule on both averages.

What Survives All That

Once you have controlled for the six, the numbers that hold up best across dates are the relative ones: dispersion width, the gap between two clubs, the difference between two shafts tested in the same block, and strike location patterns. Those are robust because whatever the conditions did, they did it to both sides of the comparison.

Absolute carry is the least portable number on the sheet, which is unfortunate because it is the one everybody wants to quote.