Do Early Risers See Less Clean Sheets?

Non league Luton Town equalled a near 100 year old record by playing in the FA Cup sixth round on Saturday. Their feat was overshadowed on two fronts. Firstly, Luton had been members of the top tier of English football as recently as 1992, so their presence and progress in England's premier cup competition was hardly unusual. The thought of Luton as a non league side still requires a quick mental double check for many football fans. And secondly, by pairing them with Millwall, media coverage inevitably focused on their notorious meeting in 1985 when football violence was at it's most extreme. Understandably, the police insisted on an early kick off time, reasoning that crowds are more docile earlier in the day. This tactic, combined with a massive police presence and a present nearly thirty years removed from the dark days of the mid '80's led to fewer than ten arrests.

Read Sean Ingle's excellent piece on the 1985 game here.

The game last weekend went largely to form. Visiting Millwall made their vastly superior league position count, as a late goal from N'Guessan added to two first half goals to give the Lions a comfortable passage to the quarter finals. The final 3-0 scoreline, however, did little to settle the debate surrounding early kick offs in general. One season can see such games produce soporific, goal droughts (in unrelated news, Fulham entertain Stoke in next Saturday's early match), while at other times, early risers are treated to a goal feast.

If kick off time is a component of how goal laden games are likely to be, it is unlikely to be the major factor. The quality of each side greatly determines the amount of goals you are likely to see scored, with the number rising, albeit gradually, as the game becomes more lopsided. If you pitch a typical Premiership title winning team at home to a typical relegation candidate, then the chances of seeing three or more goals in the game rises well above 50%. But reverse the venue and allow home advantage to bring the sides closer together in terms of ability and the possibility of three or more game goals recedes to below 50%.

So it is desirable to include a component for team quality in any study undertaken to try to isolate factors which may influence match goal totals. I've used each team's success rate over the previous 34 games to account for team quality, together with the proportion of the day which has already elapsed at kick off time for every Premiership match played over the last five completed seasons. Typically EPL games have just greater than 2.5 goals scored on average, so a sensible choice is for two goals or less to represent low scoring matches and three goals or more for high scoring ones.

Respective pregame, team quality does provide a statistically significant indication of whether game with tend towards the higher or lower end of the scoring spectrum and confirms that more lopsided games produce more overall goals. If we now add the component of kick off time, it does appear to tweak the prediction towards earlier kick offs having slightly more goals. A noon day kick off between a typical top six side hosting a mid table team sees about 54% of games go over two goals compared to just 51% of games kicking off at 8 pm. These results are consistent with results from Omar's 5 Added Minutes blog. However, as Omar also found, the time component of the regression isn't statistically significant. The effect is very likely due to chance.

It is only when we begin to look at clean sheets and start time that we may see an effect of the widely accepted cause of differing levels of athletic capability being associated with different times of the day. Clean sheets are intimately connected to team ability and even a standard Poisson approach to expected goal rates generates a reasonable fit to reality.

A regression of the pregame quality of each opponent produces similar estimates for the frequency of clean sheets in matches, but adding a coefficient for kickoff time alters that frequency around the average. Early kickoffs are less likely to see a home clean sheet than are later ones, even after team quality is accounted for. Unlike total goals, in the case of home clean sheets the kickoff time coefficient is statistically significant. The effect is unlikely to have arisen through chance.

If the effect is real, it is tempting to attempt to explain why it arises. Early kick offs tend to be televised proportionally more often for the domestic market, so a player may be keener to claim a late consolation goal in a losing cause to impress the watching millions. Alternatively, footballing skills have been shown to be more developed later in the day, so we may be seeing an effect that tells us something about the interplay between defensive and attacking play. Or it may just be a quirk of this sample of nearly 2,000 matches.

An athlete's daily body clock undoubtedly influences performance. The slow starting "morning" teams from the NFC West visiting a "mid afternoon" Atlanta team in the NFL playoffs, provided a graphic illustration of the wider, general struggles experienced by teams crossing multiple time zones to fight their battles. EPL teams never experience a domestic time zone premium, but two teams contesting a match where both are at their athletic peak, may produce a slightly different type of contest to one where both are nearer to their daily trough. And that difference may be picked up in the final scoreline.