Across the history of Test cricket, batting averages have generally risen quite steadily. The transformation - engendered by a number of factors but most significantly the evolution of pitch preparation - is most clearly illustrated by the increase in batting averages by century from 21.28 in the 19th century to 29.83 in the 20th century and now 31.80 in the 21st century.

In 2018 something curious is happening: across 25 Test matches the aggregate batting average of 24.88 is the lowest in any year since 1956 (62 years ago), the 25th lowest in any year overall and the fourth lowest in any year when there have been more than 10 matches played.

Of course, we are only two-thirds of the way through the year and a sample size of 25 matches is relatively small. However, although the drop off from 30.76 in 2017 to 24.88 in 2018 is a significant one, averages have in fact been on a downward trend since 2015, falling from 34.25 in 2014. While 2018 may be extreme, it is an embodiment of a more general pattern that is challenging the seemingly inexorable rise of batting averages in Test cricket.

The first thing to acknowledge is that batting averages are falling not because batsmen are scoring more slowly, but because they are being dismissed significantly more often. Indeed, the balls per dismissal of 49.8 in 2018 is the lowest in any year in Test history when balls faced have been recorded. Between 2015 and 2018 the balls per dismissal of 59.6 is 9.77% lower than in the 15 years previously.

Shot-type analysis shows that batsmen - although positive - have not been unusually attacking in recent years.

The change has instead come from the decline in the effectiveness of defensive shots. Between 2008 and 2014 batsmen played an average of 79.3 defensive shots per dismissal, between 2015 and now that has fallen by 21% to 62.4 and in 2018 alone to a record year low of 44.7. This trend is especially pronounced among top seven batsmen, falling by 23% from 91.2 to 69.8. Lower order batsmen’s defensive shots per dismissal has fallen by 11% from 45.6 to 40.5.

There are two possible explanations for the decline in the effectiveness of defensive batting: one is that batting has become more difficult due the conditions assisting bowlers more and/or bowlers becoming better and two is a deterioration of defensive batting techniques.

Ball tracking data shows that globally since 2015 pace bowlers have extracted slightly more seam movement and spinners have found significantly more spin.

Specifically, in Tests in Asia and the Caribbean, the ball has turned a lot more for spinners.

In Tests in England and South Africa, the ball has seamed a lot more for pacers. Conditions in Australia and New Zealand have remained relatively constant.

In the last three-and-a-half years most Test pitches have undoubtedly become more difficult for batting. The quality of bowling also appears to have improved with bowlers becoming more effective at exploiting the helpful conditions. Ball tracking data shows that both pacers and spinners are bowling more accurately since 2015 and in 2018 in particular.

An analysis of the ICC Test bowler rankings also suggests that there is an unusually high number of elite bowlers currently. On May 1, 2018, there were 17 bowlers with at least 700 rankings points - the highest number of such bowlers at the start of any month since 2006.

One theory to explain declining defensive techniques is that the growth of white ball cricket - especially T20 - in the last 15 years has caused batsmen to place greater emphasis on attacking batting, thereby neglecting their defensive games. This is a difficult thing to quantify but it cannot be denied that one-day batting has been transformed in the 21st century and to expect that the changes in technique, mindset and approach engendered by that transformation have not affected Test cricket would be naive.

One thing that is clear is that as conditions have become more extreme, it is away batsmen who have suffered the most in unfamiliar surroundings and it is the decline of away averages especially, that has dragged aggregate averages down. Since 2015 away batsmen averaged 28.71 compared to 30.53 in the seven years previously and in 2018 away batsmen average just 21.90 - the thirteenth lowest away average in Test history and the lowest in a year with more than five Tests. Since the start of 2015 away teams have lost 55% of Tests, well above the all-time average of 41% and a key reason for this is declining away batting averages.

There appears to be no easy solution to the struggles of away teams. One theory is that the toss should be scrapped and the away team empowered to choose whether to bat or bowl first. In theory, this makes sense but a similar rule - where the away team can choose to bowl first or have a toss - has made almost no difference to home advantage in the County Championship in England since the rule was implemented in 2016, increasing the win-loss ratio of away teams by 0.03.

Sufficient preparation - tour matches and practice in the relevant conditions - is regularly cited as key to the success of teams in away series. However, the scatter graph below, plotting the batting average of away teams in Test series of three matches or more against the number of first-class overs faced by that team in advance of the series shows there is, in fact, no connection between the quantity of match practice and batting performance. It can be assumed that the techniques and skills required to succeed in certain conditions are more familiar and therefore better executed by home teams than away teams but it doesn’t appear that these skills can be sufficiently acquired and improved in the weeks before the series.

Batting in Test cricket at the moment is a seriously tough task and there’s no end in sight. In the short-term at least, if conditions remain as difficult as they have been for the last few years then expect ball to continue to dominate bat and expect home teams to reap the benefits.

Data provided by cricviz.com - the world’s leading cricket analytics company.

(Freddie Wilde is an analyst at the cricket data analytics company CricViz. He tweets @fwildecricket)