The widely held view is that the top teams tend to tighten up their defences during the latter part of the season as they chase a Champions League place or look to cling onto a title winning position. But is this really the case? This analysis looks over the last 10 seasons to find the real story behind clean sheet trends.
A Decade of Clean Sheets
HeelsOverHead’s community article on clean sheets, in addition to Chris Glover’s piece from a couple of years, back prompted me to have a more detailed look at the clean sheet stats over the last 10 seasons.
What I found is that the widely held view that the stronger teams tend to tighten up at the back later in the season does not definitively bear up to scrutiny.
Consider these two charts of average numbers of clean sheets below (click image link here to enlarge) picked up match-by-match over the last ten seasons (each bar represents one match in a 38-match season. The black trend lines may just confuse matters as the bars do jump around somewhat, but I’ve left them in anyway in an attempt to capture any trend that might exist):
The chart on the left shows the average clean sheets picked up by all 35 teams in the league for the last 10 seasons, while the chart on the right focuses only on those teams that finished in the top four of each season.
Clearly the top four teams have a better overall clean sheet record but what I’m interested in here is the trend across the season – even if we ignore the 38th game of the season, which tends to be a lottery depending on which teams (if any) have anything to play for, the bias towards more clean sheets in the second half of the season is not immediately apparent.
The below image is another way of looking at the stats (click image link here to enlarge), with clean sheets earned in the first half of the season plotted on the vertical axis and clean sheets picked up in the second half of the season plotted on the horizontal axis. The black 45 degree line marks the point where a team has earned the same number of clean sheets in each half of the season.
As above, this covers the last 10 seasons from 2004/05 to 2013/14 inclusive, and I’ve shown all 35 teams (amounting to 200 data points) in the left-hand chart and just the teams who finished top four in the right-hand chart (40 data points):
If the general rule of thumb is that the top teams tighten up in defence in the second half of the season, you would expect to see a lot more data points in the bottom right half of the ‘top four’ chart.
With 40 data points over 10 years, what we actually see is this:
Seasons with more clean sheets in first half of season 18
Seasons with more clean sheets in second half of season 16
Equal clean sheets in each half of season 6
Last Two Seasons
Before we reject the prevailing view out of hand, let’s take a look at just the last two seasons for the top 4 (2012/13 and 2013/14).
Clearly this image (click image link here to enlarge) shows a much greater skew towards the second half-season than the 10-season view, with only one team (Chelsea in 2012/13) out of eight coming away with more clean sheets in the first half of the season than the second.
Perhaps the last two seasons mark some kind of trend-setting change in favour of top-four defensive strength post-Christmas? You could probably come up with all manner of reasons for why such a trend might have manifested itself. However, I’m inclined to think this can be attributed more to statistical randomness, and that reversion to the mean, as better demonstrated by the 10-season view, will kick in before too long.
Individual Teams
What about individual team records over the last 10 seasons? Let’s pick out a few of the perennial top-four candidates.
Chelsea
The west London side is arguably the one club people have in mind when they talk about teams tightening up after Christmas.
This image shows that over the course of 10 seasons we have seven seasons with a better first-half record on clean sheets and three seasons with a better second-half record. Even under Mourinho’s tenure the record is somewhat mixed, showing two seasons with a first-half skew (2004/05 and 2005/06) and two with a second-half skew (2006/07 and 2013/14).
Manchester United
For the Red Devils there is a 4-4-2 mix of first half better/second half better/both halves equal, according to this image.
Arsenal
At first glance Arsenal look like more of a candidate for a second half rush of clean sheets, but according to this image of their clean sheet performance they have averaged 6.7 in the first half-season compared to 7.9 in the second half, a difference that is not really enough to hang an entire theory on.
Everton
Statistically, the best record for second-half clean sheets of all the teams currently in the league is Everton, a record that may have come to an end now that Roberto Martinez (and his reputation for defensive frailties) has succeeded former boss David Moyes. Click here for image.
Notes:
- All raw data from the Premier League.
- Chart screenshots are from my own Tableau visualizations which can be found here.
9 years, 5 months ago
Excellent work mate. This must have taken a great deal of effort. I salute you.