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This blog is a longer (and revised) version of my poster at the OptaPro Sports Analytic forum 2015, held in London on 5th February 2015.

Introduction
When a team is goal down/up, football managers may change tactics/formation in order to protect the advantage or chase the match. Changes are more likely to happen at the beginning of the second half, or in the last quarter of a match. In the former, there is usually a change of tactics when a team is losing. In the last quarter,  substitutes are usually introduced to either hold onto a winning score, or chase a losing game. Also, if a team is winning, its manager may decide to settle for a draw, and change tactics accordingly.

In a live brodcast, match commentators usually point out a change of tactics  when it happens, or during a post-match video analysis. In contrast to this traditional method (by video), this poster aims to discover any change of tactics solely by analysing match data.

We are not aware of anyone using match data for such analysis . Our original intention was to use match data, as provided by Opta’s f24 feed, and player tracking data from TRACAB. Unfortunately attempts to use TRACAB data were not successful, and after much trying we decided not to use it.

Data and methods
The match analysed is the Newcastle-Hull played in in the 2013-14 season. The analysis was carried out using statistical and graphical (visual) methods. The statistical analysis  relies heavily on the technique of classification and regression trees.  Both the statistical and visualisation software used are designed for an interactive type of analysis, and therefore not particular suited for creating a report/poster. Despite our efforst, the clarity of the analysi may somewhat affected by this choice of tools.  MS Excel was also used for data preparation, as well as to create some charts.

To discovery tactical changes, we try finding significant changes in performance by the two teams in the many binary contexts of the game; for example between 1st & 2nd half, before/after a goal conceded/scored, before/after substitutions, etc. We also split the match in ten segments, and look at changes in activity (ball touches) between one time interval and the next,

The changes that we looked for were:
1. Change in players’ position following a goal (for/against)
2. Change of role/position of substitutes compared with starting players
3. Change in activity (ball touches) by the teams during the course of the match
4. Change in activity in the final 3rd

To identify changes, the following variables were added to the Opta data:
1. Goal_T = G_0-0, G_0_1, etc., to identify time segments (game states) when the score was 0-0, 1-0, etc.
2. Final_3rd= 1 Final_3rd ball touches (0=all other ball touches)
3. Xo, Yo = (0,100) coordinates of pitch position of ball touches

Ideally, such analysis should look  at the performance of both teams. But because of time (and space) we focus mainly on Newcastle’s performance.

Analysis results

Newcastle-Hull 2-3 (2-1)
Goals: Remy(N) 9’, Brady(H) 25’, Remy(N) 43’, Elmohamady(H) 47’, Aluko(H) 75’

Opta Poster Fig0Fig. 0  The above is a plot of main events (goals) in the context of a timeline of ten time intervals of ball touches

Fig.1 Possession (ball touches) comparison by Team and goal-time intervals
Opta Poster Fig1Fig. 1 Comparing Newcastle and Hull ball touches, we find we find that their average X_o position (pitch length) is significantly different (SD). Newcastle has more possessio in the attacking half (62.77%), and this increases to 70.91%  after Hull draws 1-1 (G_1-1) but falls as soon as Newcastle goes ahead (G_2-1) again.

Fig.2 Possession (ball touches) comparison by Role and game-state intervals
Opta Poster Fig2Fig. 2 show the increased activity of Newcastle’  midfield, from 41.92% to 54.61% ,after Hull’s draw Hull (2-2), and its steep decline by their Fwds after going ahead (2-1).

Fig. 3 Average X position comparison
Opta Poster Fig3Fig.3 Here we analyse the average X position of both teams: we find that Newcastle’s one was closer to the halfway line than Hull (SD), sign of a more attacking stance. Revealing of tactics is the position of Newcastle subs: they line up with the Full Backs (Wing backs, in fact), a defensive position they kept until Hull went ahead (2-3).

Fig. 4 Ball touches in the Final 3rd
Opta Poster Fig4Fig .4 Here we compare ball touches in the Final_3rd. Overall Newcastle was significantly (SD) more active in the final 3rd, 25.1% vs. 18.63%, but the its Subs were below the team average. Goals for/against did not result in any significant change of activity in the final 3rd. However Newcastle Subs were active mainly in the centre, in contrast to the centre-right position of the forwards they replaced.

Fig. 5 Subs vs. replaced players – comparing ball touches position
Opta Poster Fig5Fig. 5 should be viewed in conjunction with Fig. 6, and maps the position of ball touches by the Subs vs. the players they replaced.

Fig. 6 X position – Subs vs. starting players
Opta Poster Fig6Fig. 6 Here we compare Newcastle Subs average position (X) vs. tthat of the players they replaced.  We find that they took a more defensive position (SD).

Fig. 7 Significant changes in position by some Hull players – 1st vs. 2nd half
Opta Poster Fig7Fig.7 The position of ball touches of these two Hull players is significantly different in the 1st and 2nd half, which strongly suggest a change of tactics in the 2nd . The stats analysis in Fig. 8 is confirmed by these maps.

Fig. 8 Significant changes in position by some Hull players – stats
Opta Poster Fig8Fig. 8 this graph confirms statistically the intuition from Fig. 7. Both Hull goal scorers: Elmohmady (2-2, 47’), and Aluko (2-3, 75’, changed their positions significantly in the 2nd half). Aluko moved from left to right (Y), and Elmohamady forward (X).

Fig 9. Final 3rd ball touches by game state (score) interval
Opta Poster Fig9Opta Poster Fig9_1
Fig. 9 shows a map of ball touches in the Final 3rd by the two teams taken during each game state (score) interval. A chart comparing the stats count is shown above

Fig. 10 Final 3rd ball touches by Half
Opta Poster Fig10Fig. 10 Final 3rd ball touches – the black horizontal line indicates the average position of ball touches in each half. Hull’ average changes from left to right (significantly, as shown in the following graph), while Newcastle’s stays roughly the same.

Fig. 11 Final 3rd ball touches – Vertical positional shift by Half
Opta Poster Fig11Fig. 11 Final 3rd average Y position – The graph shows that Hull changed its attacking direction from the left to the right in the 2nd half (period_id) – this change was statistically significant( SD). In contrast, after an initial switch, Newcastle kept to a central position for the rest of he match.

Conclusions
The analysis identified some significant changes in performance during the match which suggest that a change of tactics may have taken place. In particular, tactical changes by Newcastle appear to have taken place after their subs were introduced. Ahead 2-1, and with 25min of the match left to play, Newcastle subs took a more defensive position then the player they replaced. In contrast, Hull started the 2nd half by increasing its attacking effort and shifting its direction from left to right; a move that quickly resulted in a goal.

Clues to the above summary conclusion were given by the following results:

 Newcastle played significantly (SD) forward than Hull and dominated possession (ball touches) throughout the match, in particular after Hull’s first goal. It was dominant in the final 3rd. Despite of this advantage, Newcastle created fewer chances than Hull, and lost the match
 The results suggest that Newcastle did not try hard enough to win the game. Subs introduce at the 65’ when the match was finely poised at 2-2 , took a more defensive stance than their predecessors, and lined up with the full backs (wing backs). They only took a more forward position when Hull scored the winning goal – too late to change the result.
 Judging by their substitutes performance, Hull appeared to do more to win the game. Their average ball touch position of their subs was equal to that of the Forwards in whole match.
 Graphical analysis show s what appears to be a change of tactics by Hull in the second half with Aluko (moving from left to right), and Elmohamady playing more forward. The latter position may help to explain the defensive ball touches of Newcastle subs Guffron and Marveaux in that area of the pitch. It is likely that they were kept busy stopping the threat posed by these Hull players in their left defensive side of the pitch.

(Note: As many would have realised, this analysis is not complete. There are a few other aspects of the match that could have been studied, and could have probably shed more light on the if/when/how changes of tactics in the match occurred. However, the objective of this post was mainly to demonstrate how a data-driven analysis with statistical and visual methods could give a more objective view of changes of formation/tactics in a match than one surmised solely by video analysis.)

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