Britain's athletes picked up only six medals at the London Stadium in August, hitting the lower end of UK Sport's target of six to eight.
Those included two for Mo Farah, who won 10,000m gold and 5,000m silver, as well as medals for all four relay teams on the final weekend.
That didn't tell the entire story, however, with five athletes finishing just off the podium in fourth place - the highest tally of any country at the championships.
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"It's brutally hard to win an athletics medal," said de Vos.
"There were I think 47 different countries that won medals, it's a genuinely global sport.
"Winning a gold medal in athletics is without doubt the hardest sport to win anything in.

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"That's the landscape, it's not a closed league of 16 teams, it's an open league of 260 countries. It's very tough."
Despite hitting UK Sport's target, albeit it at the lower end, British athletes were still down on the seven medals achieved at the 2015 World Championships in Beijing.

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It led four-time Olympic champion Michael Johnson to question the depth of British talent, while London 2012 gold medallist long jumper Greg Rutherford called for coaching improvements.
But neither are views de Vos shares, insisting the British system is working exactly as it should be.

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"Have we got people coming through? Absolutely," de Vos said. "I don't see this connection others are trying to make between not winning medals and being poorly coached.
"If you're in the top eight in the world in any track and field event, you're well coached.
"It just wouldn't happen any other way. The vast majority of those are coached locally by British coaches that come from their club system so evidentially there is a system that's working."
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