Baseline Context Addendum: Tatum vs Brown

Companion to celtics_tatum_brown_prosperity_analysis.md. Same BOS-PHI 2025-26 first-round series, same data sources, same player-tagging logic. Adds four comparison contexts so the prosperity numbers can be read against each player's own baseline in the series.

Source: data/celtics_tatum_brown_all_events_2025_26.csv (428 tagged rows across all 7 games) and data/celtics_tatum_brown_baseline_context_summary.csv.

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Context definitions

All five contexts are computed twice: ender_only (cleanest, one row per possession the player finished) and all_roles (includes assist-creation credits, double-counts Tatum-to-Brown threes etc).

Ender-only summary (cleanest comparison)

Jayson Tatum

Jaylen Brown

All-roles summary (includes creator double-credit)

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Q1. Does Tatum get worse in prosperity vs his own baseline?

Partially. The scoring is up but the shot mix and turnover rate degrade.

vs full-series baseline (ender-only):

• PPP: 1.074 vs 0.933 (+15%). Better.
• Turnover rate: 18.52% vs 11.33% (+63% relative, +7.2 pp). Worse, by a lot.
• Rim rate: 18.52% vs 22.00% (-3.5 pp). Worse.
• Force rate: 40.74% vs 43.33% (-2.6 pp). Slightly worse.
• 3PA rate: 33.33% vs 34.67%. Essentially unchanged.

Reading: Tatum is hunting buckets effectively in prosperity (1.074 PPP is fine), but he is feeding the opponent. The TO surge from 11% baseline to ~19% in prosperity is the single largest within-player swing in the dataset, and it is exactly the kind of event that gives a trailing team life.

Q2. Does Brown get worse in prosperity vs his own baseline?

No, not on the headline numbers. He gets more efficient and turns it over less. The settle-rate goes up but it was already high.

vs full-series baseline (ender-only):

• PPP: 1.071 vs 0.841 (+27%). Much better.
• Turnover rate: 7.14% vs 11.68% (-4.5 pp, -39% relative). Better.
• Midrange rate: 35.71% vs 27.10% (+8.6 pp, +32% relative). Worse.
• Rim rate: 17.86% vs 20.09% (-2.2 pp). Worse.
• Force rate: 42.86% vs 41.59%. Essentially unchanged.

Reading: Brown in prosperity scores more efficiently, protects the ball more, but settles more often for midrange. The pre-existing tendency toward pullup twos amplifies. He is not the player creating Stevens's "let them back in" moments, but he is the player making Stevens's "lacks rim pressure" pattern visible on the shot chart.

Q3. Are they both worse?

No. Both have a clear shape change in prosperity, but only Tatum gets worse on a net-scoring basis once you count the cost of his turnovers.

If you charge each TO at the league-average value of a possession (~1.1 points), Tatum's PPP-after-TO-cost in prosperity is roughly 1.074 minus (0.185 * 1.1) = ~0.871. Brown's PPP-after-TO-cost is roughly 1.071 minus (0.071 * 1.1) = ~0.993.

Adjusted view (rough, but defensible):

Brown's prosperity possessions are net more valuable to Boston than Tatum's once turnovers are priced in. This is a directional finding on a small sample, not a settled one.

Q4. Which failure mode is more abnormal relative to baseline?

In absolute percentage points, Brown's midrange settle is the larger shift (+8.6 vs +7.2 pp). In relative terms (how far each player has moved from his own normal behavior), Tatum's turnover surge is the larger shift (+63% vs +32%).

The more abnormal failure relative to baseline is Tatum's turnovers. Brown's midrange diet is a pre-existing condition. Tatum's prosperity TO rate is a state change.

A subsidiary signal that supports the same read: in non_prosperity_pressure (tied or trailing by 5 or less), Tatum took 8 events and Brown took 22. Tatum's 8 events were 50% threes (4 of 8, 1 made, PPP 0.375). When Boston is desperate, Tatum hunts deep threes at poor efficiency. Brown carries far more of the volume in losing situations and is also inefficient (0.591 PPP), but he is on the floor trying to produce shots. The pressure-context sample is tiny (30 events combined) but it reinforces the within-player pattern: Tatum's behavior changes more by game state, Brown's behavior is mostly constant.

Q5. Verdict

Sample too small for proof. Signals lean toward "Tatum is the prosperity-specific problem; Brown is the pre-existing problem."

The evidence:

• The clearest prosperity-specific behavioral shift in the dataset is Tatum's turnover rate jumping from 11% baseline to 19% in prosperity. That is consistent with Stevens's "let the opponent back in" mechanism.
• Brown's prosperity behavior is a slightly worse version of his already- present settle-for-midrange habit. It contributes to the lack of rim pressure Stevens described, but it is not a behavior that appears when Boston gets a lead. It exists in every context.
• On net possession value once turnovers are priced in, Brown's prosperity possessions outperform Tatum's (~0.99 vs ~0.87 effective PPP).
• The non-prosperity pressure data, despite tiny samples, says Tatum becomes the most three-happy in losing situations (50% 3PA rate, 1 of 4 made). That is the same "settle from distance" pattern people more often attach to Brown.

Caveats that prevent calling this a verdict:

• 55 ender-only prosperity events (27 Tatum, 28 Brown). One series.
• Three of Boston's four losses had zero prosperity sequences because Boston never built the lead. The Stevens narrative may be drawing on one game (Game 5) plus seasons of memory the data here does not yet cover.
• "Initiator" cannot be measured from public PBP. A Brown-finished possession that Tatum created with a drive-and-kick gets credited to Brown only.

The case to extend before publishing as a verdict: run the same five contexts against the 2024-25 NYK second-round series (game ids 0042400211 through 0042400216) and the 2023-24 finals run. If Tatum's prosperity-specific TO surge replicates and Brown's midrange constancy replicates, the directional read here becomes a verdict. Both the discovery list (data/celtics_playoff_games_discovered.csv) and the parameterized script (scripts/build_celtics_baseline_context.py) are set up for that extension.

Bottom line for a 2-line message-board take

In the 2025-26 series, Tatum's turnover rate jumped from 11% baseline to 19% only when Boston had a lead, while Brown's midrange-heavy diet was nearly identical in every context. If you charge turnovers at league value, Brown's prosperity possessions were worth more to Boston (~0.99 PPP) than Tatum's (~0.87 PPP). Small sample, one series, but the prosperity-specific failure pattern lines up with Tatum and the always- on settling pattern lines up with Brown.