Question: How often is the MacRobertson International Croquet Shield tournament?
About the article "Croquet", section "Association".

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The MacRobertson International Croquet Shield is the premier croquet teams event in the world.
It is currently competed for by Australia, England, New Zealand and the United States.
It is known affectionately as the "MacRob" or just the "Mac".
The series is now played in rotation between the competing countries every three or four years.
The next series will be held in 2017 in the USA. 
There are several variations of croquet currently played, differing in the scoring systems, order of shots, and layout (particularly in social games where play must be adapted to smaller-than-standard playing courts).
Two forms of the game, association croquet and golf croquet, have rules that are agreed internationally and are played in many countries around the world.
The United States has its own set of rules for domestic games.
More unusual variations of the game include mondo croquet, extreme croquet, and bicycle croquet (perhaps influenced by polo).
Gateball, a sport originated in Japan under the influence of croquet, is played mainly in East and Southeast Asia and the Americas, it can also be regarded as a croquet variant.
As well as club-level games, there are regular world championships and international matches between croquet-playing countries.
The sport has particularly strong followings in the UK, US, New Zealand and Australia; every four years, these countries play the MacRobertson Shield tournament.
Many other countries also play.
The current world rankings show England in top place for association croquet, followed by Australia and New Zealand sharing second place, with the United States in fourth position; the same four countries appear in the top six of the golf croquet league table, below Egypt in top position, and with South Africa at number five.
Croquet is popularly believed to be viciously competitive.
This may derive from the fact that (unlike in golf) players will often attempt to move their opponents' balls to unfavourable positions.
However, purely negative play is rarely a winning strategy: successful players (in all versions other than golf croquet) will use all four balls to set up a break for themselves, rather than simply making the game as difficult as possible for their opponents.
At championship-standard association croquet, players can often make all 26 points (13 for each ball) in two turns.
Croquet was an event at the 1900 Summer Olympics.
Roque, an American variation on croquet, was an event at the 1904 Summer Olympics. 
In the early 1920s, Australian confectionery maker and philanthropist, Sir Macpherson Robertson played croquet as his pastime.
He enlisted returned World War I veterans to take up concessions for his products and encouraged them to try croquet also, believing it to be a teaching aid for developing skills in self-discipline and risk management.
Eventually, a pool of players were participating in regular tournaments.
In 1925, with a growing interest in the sport, he established the MacRobertson Shield, an international tournament of croquet "Tests" between Australia and England.
The Victorian Croquet Association sent a team of four players to England to contest the inaugural tournament.
England won easily and Robertson presented the Shield personally.
In 1927-8 an England team played in Australia and was narrowly defeated.
In 1930, New Zealand was invited to participate in a series between themselves and Australia, with Australia winning comfortably and the first triangular series was held in 1935 in Melbourne as part of the Melbourne centenary celebrations.
In 1993, a United States team was admitted into the tournament.
Between 1974 and 2010 the team representing the Croquet Association was named Great Britain and in two series Great Britain and Ireland, because the teams included players from Scotland, Wales and Ireland.
From the 2013-14 series the team will be named England and will only included English qualified players.
This is following the introduction of the WCF World Association Croquet Team Championship. 
The 2010 Shield was contested in Great Britain, across 5 different venues.
"Fri 6 – Tue 10 August": Great Britain v USA at Bowdon, Greater Manchester and Australia v New Zealand at Heaton Park, Manchester"Thu 12 – Mon 16 August": Great Britain v New Zealand at Nottingham and Australia v USA at Roehampton"Wed 18 – Sun 22 August": Great Britain v Australia and New Zealand v USA both at SurbitonEach team is represented by 6 players and each test is the best of 21 matches comprising 12 singles and 9 doubles.
Also in 2010, for the first time, the 4 MacRobertson Shield countries were joined by 13 other countries in an inaugural WCF World Association Croquet Team Championship.
The MacRobertson Shield was Tier 1 and the other 13 entries were split into three further tiers based on playing strength.
"Mon 16 - Sat 21 August: Tier 2 Division 1": The next four countries played at the Hurlingham Club, London.•Tier 2.1 winner Wales "Mon 16 -Sat 21 August: Tier 2 Division 2": The next four countries played at the Compton Croquet Club, Eastbourne, East Sussex.•Tier 2.2 winner Scotland "Wed 18 - Sat 21 August: Tier 3": The remaining five countries played at the Sussex County Croquet Club.•Tier 3 winner Czech Republic 
Dudley James Vincent Hamilton-Miller (28 March 1904 – 15 July 1984) was a croquet player from England.
Dudley Hamilton-Miller won the Open Championship twice (1938 and 1946) and the President's Cup once in 1946.
As an administrator, Hamilton-Miller served on the Council of the Croquet Association between 1966 and 1983.
Hamilton-Miller represented England in the 1950–51 MacRobertson Shield tournament in New Zealand.