Post-Shot xG: Advanced Expected Goals Metric

Introduction

Post-Shot xG (PSxG) is an advanced expected goals metric that evaluates shot quality after the ball leaves the player's foot, accounting for shot placement, power, and trajectory. While standard xG measures pre-shot probability, PSxG provides a more accurate assessment of goalkeeper performance and finishing quality. This guide explains PSxG football metrics, calculation methods, and practical applications.

What is Post-Shot xG?

Definition

Post-Shot xG (PSxG): The probability a shot results in a goal, calculated after considering where the shot was aimed and its characteristics (speed, trajectory), but before the goalkeeper's save attempt.

Key Difference from xG:

• xG: Probability before shot is taken (based on position, angle, etc.)
• PSxG: Probability after shot is taken (based on shot placement quality)

Visual Example

Scenario: Penalty Kick

Pre-Shot (xG):

Position: Penalty spot (12 yards, central)
Situation: Penalty
xG: 0.79 (79% historical conversion rate)

Post-Shot (PSxG):

Shot Placement: Top right corner
Shot Power: 70 mph
PSxG Options:
- Top corner (unsaveable): PSxG = 0.98
- Mid-height center: PSxG = 0.50
- Low corner (saveable): PSxG = 0.85

Interpretation: Same initial xG (0.79), but PSxG varies based on shot quality.

How Post-Shot xG is Calculated

Required Data

Tracking Data:

1. Shot Coordinates: Where the ball crossed the goal line (or would have)
2. Shot Velocity: Ball speed in mph/kph
3. Shot Trajectory: Dipping, rising, straight
4. Goalkeeper Position: Location when shot was taken

Calculation Process

Step 1: Determine Target Location

Divide goal into grid:

Goal divided into 9 zones:
Top Left | Top Center | Top Right
Mid Left | Mid Center | Mid Right
Low Left | Low Center | Low Right

Step 2: Historical Analysis

For each zone, analyze thousands of historical shots:

Top Right Corner:
- Total shots aimed there: 2,500
- Goals scored: 2,350
- PSxG for this zone: 2,350 / 2,500 = 0.94 (94%)

Step 3: Adjust for Shot Power

Powerful shot (75+ mph): +0.02 PSxG (harder to save)
Average shot (60-75 mph): No adjustment
Weak shot (<60 mph): -0.03 PSxG (easier to save)

Step 4: Adjust for Goalkeeper Position

Goalkeeper well-positioned: Standard PSxG
Goalkeeper off-line: +0.05 PSxG (harder to save)
Goalkeeper diving early: +0.08 PSxG

Final PSxG:

Base (top corner): 0.94
+ Shot power bonus: +0.02
+ GK positioning: +0.03
= Final PSxG: 0.99

PSxG vs xG: Key Differences

Comparison Table

Aspect	xG	PSxG
Timing	Before shot	After shot, before save
Factors	Position, angle, defenders	Shot placement, power
Purpose	Evaluate chance quality	Evaluate shot quality + GK performance
Use Case	Team/player attacking quality	Finishing ability + GK saves
Accuracy	~70-75%	~80-85%

Example Scenario

Mohamed Salah Shot:

Pre-Shot (xG):

Distance: 18 yards
Angle: 15° (slightly off-center)
Situation: Open play
Defenders: 1 in vicinity
xG: 0.18 (18% chance)

Post-Shot (PSxG):

Shot Placement: Bottom right corner
Shot Power: 68 mph
Goalkeeper Position: Slightly off-line
PSxG: 0.75 (75% chance)

Outcome: Goal

Analysis:

• xG (0.18) suggested low-quality chance
• PSxG (0.75) revealed excellent shot placement
• Salah elevated a mediocre chance into a high-quality goal
• Conclusion: Elite finishing

Applications of Post-Shot xG

1. Goalkeeper Performance Evaluation

Formula:

Goalkeeper Performance = Goals Conceded - PSxG Faced

Interpretation:

Negative value: Goalkeeper saved more than expected (elite)
Zero: Average performance
Positive value: Conceded more than expected (poor)

Premier League 2023-24 Example:

Alisson (Liverpool):

PSxG Faced: 42.3
Goals Conceded: 34
Performance: -8.3 goals
→ Saved 8.3 goals above expectation (elite)

Mid-table Goalkeeper:

PSxG Faced: 58.7
Goals Conceded: 62
Performance: +3.3 goals
→ Conceded 3.3 more than expected (below average)

2. Finishing Quality Assessment

Formula:

Finishing Quality = PSxG - xG

Interpretation:

Positive value: Excellent shot placement
Zero: Average finishing
Negative value: Poor shot placement

Example: Striker Comparison

Erling Haaland:

Average xG per shot: 0.22
Average PSxG per shot: 0.35
Finishing Quality: +0.13
→ Consistently improves shot quality through placement

Average Striker:

Average xG per shot: 0.20
Average PSxG per shot: 0.20
Finishing Quality: 0.00
→ Doesn't improve or worsen chances through placement

3. Shot Stopping Analysis

Exceptional Saves:

Scenario:

Shot PSxG: 0.92 (almost certain goal)
Outcome: Saved
→ Exceptional save (saved 0.92 PSxG)

Routine Saves:

Shot PSxG: 0.15 (low-quality shot)
Outcome: Saved
→ Routine save (expected to save)

4. Team Shooting Efficiency

Team Analysis:

Manchester City:

Team xG per shot: 0.14
Team PSxG per shot: 0.18
Shot Quality Improvement: +0.04 (excellent shot selection)

Newcastle United:

Team xG per shot: 0.12
Team PSxG per shot: 0.11
Shot Quality Improvement: -0.01 (poor shot placement)

Interpretation: Man City players improve their chances through better shot placement, while Newcastle wastes opportunities with poor placement.

Advanced PSxG Metrics

1. PSxG +/-

Definition: Difference between actual goals and PSxG.

Formula:

PSxG +/- = Actual Goals - PSxG

Player Example:

Kylian Mbappé:

Goals: 29
PSxG: 25.3
PSxG +/-: +3.7
→ Exceptional finishing (scores more than shot placement suggests)

Interpretation: Mbappé scores even when shot placement isn't perfect, indicating elite finishing instinct.

2. PSxG Conversion Rate

Formula:

PSxG Conversion = (Goals / PSxG) × 100

Categories:

115%+: Elite finisher
100-115%: Good finisher
95-100%: Average
<95%: Below average

Example:

Harry Kane:
- Goals: 30
- PSxG: 28.5
- Conversion: 105% → Good finisher

3. Goalkeeper PSxG Save %

Formula:

PSxG Save % = ((PSxG - Goals Conceded) / PSxG) × 100

Example:

Ederson (Manchester City):
- PSxG Faced: 40.0
- Goals Conceded: 33
- PSxG Save %: ((40.0 - 33) / 40.0) × 100 = 17.5%
→ Saves 17.5% more than expected

PSxG Limitations

1. Requires Advanced Tracking Data

Challenge: PSxG needs ball trajectory tracking, which requires:

• Multiple cameras
• Advanced tracking software
• Expensive infrastructure

Availability:

• Top leagues: Yes (Premier League, La Liga, Bundesliga)
• Lower leagues: Often no

2. Doesn't Account for Deflections

Issue: Deflections change trajectory unpredictably.

Example:

Original PSxG: 0.20 (going wide)
Deflection: Ball redirects to top corner
Actual Outcome: Goal
→ PSxG was inaccurate due to deflection

3. Goalkeeper Reaction Time Not Factored

Issue: PSxG assumes average reaction time.

Reality:

• Elite goalkeepers: Faster reactions
• Aging goalkeepers: Slower reactions

Impact: PSxG may over/underestimate save probability.

PSxG in Practice: Real Match Analysis

Case Study: Alisson's Wonder Save

Match: Liverpool vs Brighton (2023)

Shot Details:

Shooter: Brighton Forward
Distance: 8 yards
Angle: Central
xG: 0.65 (good chance)

Post-Shot:

Shot Placement: Top right corner
Shot Power: 73 mph
PSxG: 0.92 (almost certain goal)

Outcome: Alisson saved it

Analysis:

• xG (0.65): Good chance but not certainty
• PSxG (0.92): Near-perfect shot, should be goal
• Conclusion: Exceptional save (saved 0.92 PSxG)

Impact on Goalkeeper Stats:

Alisson season PSxG performance: -8.3 goals
This save alone: -0.92 (significant contribution)

Data Sources for PSxG

Professional Sources

StatsBomb:

• Provides PSxG data for major leagues
• Requires subscription

Opta:

• Used by Premier League
• Expensive, professional use

Wyscout:

• Academic access available
• Covers multiple leagues

Free Alternatives

Limited Free Options:

• Some matches on FBref (StatsBomb open data)
• Manual estimation using shot maps

DIY Approach: Create lookup table based on shot target zones using historical data.

Conclusion

Post-Shot xG is a powerful advanced metric that evaluates shot quality and goalkeeper performance more accurately than standard xG. By accounting for shot placement and power, PSxG reveals exceptional saves, elite finishing, and shot quality improvements.

Key Takeaways:

1. PSxG accounts for shot placement, making it more accurate than xG
2. Goalkeeper performance best measured using PSxG (goals conceded vs PSxG)
3. Finishing quality revealed by PSxG - xG difference
4. Requires advanced tracking data, less available than standard xG
5. Use alongside xG for comprehensive analysis

Golden Rule: Use PSxG to evaluate goalkeepers and shot-stopping. Use xG for team/player chance creation. Combined, they provide complete offensive and defensive evaluation.

Frequently Asked Questions

How is PSxG different from xG?

xG measures pre-shot probability (position, angle). PSxG measures post-shot probability (shot placement, power). PSxG is more accurate but requires advanced tracking data.

Can I access PSxG data for free?

Limited free access via StatsBomb Open Data on FBref. Most comprehensive PSxG data requires paid subscriptions (StatsBomb, Opta).

Why is PSxG better for evaluating goalkeepers?

PSxG accounts for shot quality (placement, power), making goalkeeper evaluations fairer. A goalkeeper facing many unsaveable shots (high PSxG) shouldn't be penalized for conceding.

Who are the best goalkeepers by PSxG?

Alisson, Ederson, and Marc-André ter Stegen consistently outperform PSxG by 6-10 goals per season. They save more than shot quality suggests they should.

Does PSxG work for penalties?

Yes. Standard penalty xG is ~0.79. PSxG varies by placement: top corner (0.98), center (0.40), low corner (0.85). Helps evaluate penalty takers and goalkeeper saves.

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Meta Description: Post-Shot xG (PSxG) explained: Advanced expected goals metric for evaluating shot quality, goalkeeper performance, and finishing ability.

Keywords: post shot xg, psxg football, psxg metric, expected goals after shot, goalkeeper psxg, shot quality analysis

Category: Education

Word Count: ~1,500 words