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A dominant home record is a significant factor behind New Zealand reaching the top of the ICC Test rankings. Their overall win-loss ratio of 3.2 is easily the best from May 2017, which is the period taken for the rankings - the next-best is India's 1.9 - but at home they have been impregnable, winning 13 out of 16 Tests, and losing none. India are the only other similarly dominant team at home, with a 9-0 record in 11 Tests.
New Zealand are only seventh in terms of number of Tests played since May 2017 - their 24 Tests during this period is slightly more than half of England's 45, but they have made almost every series count. The one debacle was in Australia, when they lost 3-0. In their remaining 21 Tests, they won 16 and lost just two. In the period from May 2017 to April 2019, when the results get 50% value in the ICC rankings, New Zealand had an 8-1 win-loss record; since May 2019, their record is 8-4.
While New Zealand have undoubtedly been a formidable team with bat and ball, it is also true that they have had a favourable calendar over these 44 months: eight of their 11 series have been at home, and they have won each of those. England and West Indies have lost two series each in New Zealand during this period, while Sri Lanka, Bangladesh, India and Pakistan have all been beaten once each.
All those wins have helped rack up the points, but there have also been three crucial wins overseas which have made a difference between New Zealand actually taking the top place, and them hovering just a point or two behind the leaders. In UAE in 2018-19, they stole a 2-1 series win against Pakistan - including an incredible four-run win in Abu Dhabi - while in Sri Lanka they managed to level the series after losing the first Test.
Despite those three wins, though, New Zealand are only fourth in terms of win-loss ratios in away Tests (including neutral venues). India are on top on that parameter with a 10-10 record in away Tests, followed by Sri Lanka and England. New Zealand have played only eight out of 24 Tests overseas, and haven't played a series in England or India during this period. (To compare, India's ongoing series in Australia is their seventh overseas series in this period.)
With an average of 64.48, Kane Williamson is on top of the charts among batsmen who have scored 1000-plus runs during this period. He pips Steven Smith and Virat Kohli, who both also average over 60 during this period. It hasn't been a one-man show, though, for New Zealand have two other batsmen in the top 10: Henry Nicholls, who averages 51.50, and Tom Latham, who has averaged 47.63 at a time when openers have generally struggled. Add the contributions of Ross Taylor, BJ Watling and Colin de Grandhomme, and New Zealand have six batsmen with 900-plus runs averaging more than 38.
Bowling has always been New Zealand's strong suit, especially in home conditions, and the presence of Kyle Jamieson in their last six Tests has added even more teeth to an already powerful bowling attack. Jamieson's 36 wickets have come at an astonishing average of 13.27, while Neil Wagner and Tim Southee have 85-plus wickets at sub-23 averages. Trent Boult has been slightly off-colour recently, but his average of 26.47 is pretty good too.
Thanks to these key contributors with bat and ball, the difference between New Zealand's batting and bowling averages during this period is 12.1. It isn't the highest, though, because India have been even more dominant in their home victories. That won't matter to New Zealand, though; where they will want to be ahead of India (or Australia) is on the World Test Championship table, on which they are currently third. If they manage to make it to Lord's later this year for the final, that will cap off a fantastic four years for Williamson and his team.