Calls to dismantle the country's old boys' club have become louder after women were found to make up nearly half the country's directors associated with the best results for shareholders, despite occupying only a fifth of all boardroom seats.

The findings came after nearly 2000 board appointments, and 10 years of share returns performance of 364 NZX-listed companies, were analysed as part of the Herald's "Directors of the Decade" project.

The project identified and ranked 33 professional directors who had four or more public appointments over the period. Women made up four of the nine directors associated with high shareholder returns over the past decade.

The project resulted in Sam Knowles, whose companies enjoyed annual returns 37.4 per centage points above the NZX average, taking first place, followed by Julia Hoare (+32.2) and Gráinne Troute (+19.9).

The bottom end of the table was occupied by Mike Allen (-15.2), Keith Smith (-9.9) and Michael Stiassny (-7.3).

Professor Judy McGregor, head of AUT's school of social science, said the results didn't surprise her and called for further change in New Zealand boardrooms that historically considered established tenure as the main qualifier for office.

"Anything that demonstrates that women are helping to improve performance can only be positive, in terms of chipping away at the white male clubbiness which we still have," she said.

While business and statistics professors cautioned against reading the results as proof of causality - company performance is a complex function affected by many factors other than just corporate governance - the Herald findings suggest firms with female board members are disproportionately correlated with high returns for shareholders. (Story continues at the end of the graphic.)

You can explore the rankings in our interactive data visualisation below. Each chart shows the shareholder returns (red line) compared with the middle 50% of NZX companies (shown in grey) when the director was on the board of that company.

A report by the NZX earlier this year showed women made up just 22 per cent of board members of all companies it listed. This figure had increased from just eight per cent 10 years earlier.

The Institute of Directors' Felicity Caird welcomed the Herald results.

"To have that sort of finding actually does signal the value of having a more diverse board - at least in terms of gender."

Caird said while evidence was not yet conclusive, diversity in the boardroom was increasingly recognised as important for business.

"Our push for diversity is you need diversity of thought and experience to bring value. Having a diverse board includes women - but also ethnicity and age."

Warehouse deputy chair Keith Smith, ranked 32nd of 33 professional directors by the Herald, has recently become eligible to collect New Zealand Superannuation.

He said he had already announced his retirement from his public boards, and saw the writing on the wall. "Change is coming. I know people say it's too slow," he said.

"If you had the choice of a white male like me, or someone that's diverse in a range of areas, and we're otherwise equal? I know who they'd pick, and it wouldn't be me. And they wouldn't be wrong."

Female directors spoken to by the Herald identified appointment processes as needing improvement.

Historically, company appointment had been managed through informal recommendations and Hoare said such informal "shoulder-tapping club environment" processes limited the pool of potential candidates to a social or professional circle.

"You're just going to get the same names in the same bucket every time," she said.

A recent trend, particularly amongst larger listed firms to outsource board candidate shortlisting to independent recruitment firms, was welcomed by almost all directors and commentators but obstacles were identified in sourcing new candidates.

Hoare said the recruitment firm process was responsible for all her appointments. Sam Knowles, who topped the "Directors of the Decade" table, said it was involved in only half of his.

Troute said the recruitment firm route worked, but "that tends to only happen once you've established yourself. And that's why it's really tricky to get that first opportunity".

But Michael Stiassny, whose tenure at troubled NZ Windfarms was largely responsible for having him ranked third last by the Herald, cautioned against campaigning solely against the old boys.

"There are now various clubs. I made reference the other day to Graeme Samuel in Australia referring to the women's club," he said.

(In March, Samuels faced a torrent of criticism across the Tasman after he said there needed to be a "nuclear bomb to smash down the impenetrable wall around the female club of directors".)

Stiassny was adamant the problem wasn't gender-based.

"All of these clubs, groups, make their life easy. What that does, of course, means that diversity of thought and diversity of ethnicity, or any form, is probably going to find it quite hard to get through."

One market veteran, who preferred to remain anonymous in order to preserve professional relationships, said he broadly agreed with the Herald rankings and was firmly in favour of more women directors.

Not out of idealism, but because they were guaranteed to not be part of the Auckland-based old boys' club that dominated New Zealand boardrooms for decades.

"They tend to know each other from school years, played rugby together. It's not as bad as it was - it was a shocker - but it isn't getting better fast enough," he said.

"Women break that club up because they don't come from that background."

METHOD: How the numbers were crunched

The Herald's "Directors of the Decade" takes a broad approach rather than a deep one, seeking a metric that is easily available, consistent, and with a nationwide spread.

In this case, we analysed company shares listed on the NZX, pared with their directors' tenure as recorded on the Companies Register.

Choosing a 10-year period between Jan 1, 2009 and Dec 31, 2018, was arbitrary - although there's alliteration with "directors" and "decade" - but intended to capture the current directing pool rather than that of the past.

As it turned out, only one of our final pool - Sir John Anderson (29th/33) who died last year - was no longer active as a director at the end of the target period.

The performance metric settled on was average gross return per share, a measure looking at 12-month changes in adjusted gross share price, which takes into account stock price movements and dividend payments.

This was in turn weighted against overall NZX performance during the tenure period to try and capture whether markets had been out- or under-performed.

Further refinements were to made, to limit volatility, culling small-cap stocks and back-door-listing like Veritas.

Only markets for ordinary shares were considered, in order to exclude illiquid and flat markets in more exotic securities.

And to capture both a pool of professional directors, and ensure each subject had multiple data points to measure, those with three or fewer public board postings were ignored.

The analysis initially looked at 364 stocks, filtered to 168, and 1986 directors, titrated to our busy pool of 33.

The resulting measure, displayed in a How they Rate? table and accompanying visualisation showing director-by-director details, is simple and with obvious limitations, but such limits are easily understandable because they is simple.

It does not capture a director's private company performance, as financial information for these firms is rarely public or consistent. This means directors of recent high-profile collapses, such as Mainzeal and Ebert, unfortunately escape being measured.

It also misses out New Zealand directors' activity on companies listed elsewhere, as Mike Allen (33rd/33) was keen to point out. Shares of his Coats Group - which effectively flatlined on the NZX over the past decade - have tripled in value since migrating to the London Stock Exchange in 2016.

CORRECTION: An earlier version of this article incorrectly listed Mike Allen's directorships as including Mercury NZ. The former director of that company is Michael David Allen, not Michael Nicholas Allen, the subject of this story. The interactive graphic also included a photo of the wrong Michael Allen. The Herald apologises for the error.