The coefficient of variation is the standard deviation as a percentage of the mean average. This is effectively a measure of how spread out the number of wins is. A low percentage means that the number of wins for each club is more clumped around the average while a high percentage means that the number of wins recorded that season by each club is more spread out from the overall average – therefore implying that the division is much more uneven.
The technical bits out the way the graph shows an interesting pattern. Viewed over a time period of 1983-84 until the present the trend in the Championship appears to mirror that of the Premier League, but with a slight delay; For instance in 2010-11 to 2011-12 the Premier League saw a sharp rise in the coefficient from 33.7 to 43.8. A similarly sharp rise was then seen in the Championship between 2012-13 to 2013-14 29.6% to 34.5%
A gap appeared to open up between the two, when in 1993-93 the Premier Leagues coefficient rose to 38.1% from 23.9% of the previous season while at the same time the Championships declined from 31.4% to 23.1%
This gap was then closed in the late 90s thanks to the Championships coefficient increasing, notably between 1995-96 and 1998-99 when it rose from 20.2% to 35.6%
In the late nineties and early noughties, between 1997-98 and 2000-01 the coefficients of the two divisions were roughly in line however, from this point on the two have diverged with the Premier Leagues coefficient continuing on an upwards trajectory, while the Championships has taken a downward trajectory, where it even reached a low 19.6% in the 2012-13 season. Despite these different trajectories however, the trends seem to still be mirrored with the lines following a similar pattern, albeit on their differing trajectories.
The start of the Premier League era was associated with the top-division becoming more unequal in terms of wins and this trend fed into the Championship a few seasons later in a kind of trickle-down effect. However, over the last decade, the broad trend has been for the Championship to become more even on the field with teams recording a number of wins closer to the overall average for the division.
In terms of explanations I’m stumped. In my last post I looked at the long term trends for the top-flight going way back and my explanation there is that the general trend towards less on-field equality is driven by the reduction in the role of luck which has been an underlying feature from the development of goal-nets to the implementation of goal-line technology.
If anyone has any other hypothesis, or explanations then please feel free to add your thoughts! The data I have used is in the table below.