Referees chief Mike Riley accepts VAR has missed four errors in the Premier League this season but promised officials are "constantly learning".

The new technology was introduced at the start of the 2019-20 campaign to mixed response, intervening on six occasions across the first four matchweeks after 227 reviews.

Much of the focus has been on incidents where VAR has not had an input, however, and Riley, manager of the Professional Game Match Officials Limited (PGMOL), acknowledged four mistakes in a meeting with the Premier League on Thursday.

He told Sky Sports: "We're learning as we go along and we're constantly improving as we go along.

"Undoubtedly, out of the four match rounds, there have been some really good examples where we have intervened. We've had six instances where the VAR has advised the referee and we've got a better decision as a result.

"What we've talked about are the four instances where the VAR didn't intervene and, had we done, we would have had a better understanding of the role VAR plays in the game."

Explaining the errors, he continued: "They're all about the judgement of the VAR and the process we adopt and then discussing those with the clubs, with the players, with the managers to make sure these really are examples where the VAR could have had a benefit and intervened and helped the referee on the day.

"It's a combination of factors. That's the fascinating thing as this project evolves. We're constantly learning as we do things.

"On occasion, it was a judgement decision the VAR made, on one, it was a process mistake, and on others, it was trying to protect the high bar we want to not disrupt the flow of the game.

"On these two occasions, actually, the judgement should have been that the high bar had been crossed, it was a clear and obvious error and the VAR could have advised the referee of that."

It is reported the four incidents Riley flagged up in the meeting were Jefferson Lerma's challenge on David Silva, Youri Tielemans' apparent stamp on Callum Wilson, Fabian Schar's goal for Newcastle United against Watford and Tom Trybull's trip on Sebastien Haller.

Wanted more than a point at home!! Happy for my 1 goal of the season!! #nufc pic.twitter.com/SJyYrRkHEx
— Fabian Lukas Schär (@fabianschaer) August 31, 2019

FABIAN SCHAR GOAL

The VAR was criticised when Gabriel Jesus had a goal disallowed for City as Aymeric Laporte appeared to handle against Tottenham. A similar incident went unpunished when Newcastle drew with Watford. Isaac Hayden knocked down Emil Krafth's cross with his arm for Schar to score and earn the Magpies a point.

TRYBULL ON HALLER

Haller netted the opener as West Ham defeated Norwich City 2-0 on August 31, but the margin of victory could have been wider. The Hammers should have had a penalty when Trybull tripped Haller as he advanced into the area, but no foul was given.

LERMA ON SILVA

The first error of the season came in Manchester City's 3-1 win at Bournemouth on August 25. Lerma clearly stepped on Silva's foot inside the area, but no penalty was given to the champions.

TIELEMANS ON WILSON

The only instance in which a player should have been sent off but was not saw an awful Tielemans challenge go over the ball onto Wilson's ankle, with not even a free-kick awarded. The Belgium midfielder went on to score in Leicester City's 3-1 win over Bournemouth.

VAR check...

Tielemans went in hard on Callum Wilson's ankle, the home fans were nervous... But it's play on.

55 | 2-1 // #LEIBOU
— AFC Bournemouth (@afcbournemouth) August 31, 2019

NOTABLE NON-AWARDS

It appears the PGMOL were satisfied with several other high-profile incidents.

Pep Guardiola believed Rodri should have had a penalty against Tottenham when Erik Lamela pulled him to the ground, while Spurs themselves wanted a spot-kick as Jamaal Lascelles fell into a challenge on Harry Kane in Newcastle's away win the following week.

Gary Cahill was only shown a yellow card in Crystal Palace's shock victory at Manchester United despite appearing to be the last man as he tugged down Anthony Martial.

How about that for a debut?#CPFC | #MUNCRY pic.twitter.com/VhL7jHUtVC
— Crystal Palace F.C. (@CPFC) August 24, 2019