Most ETFs have returned about 7-8 % average long term (before allowing for inflation). Not a bad return, but very different over 20 years from 10-15%

Average rate of return is 7% accounting for inflation, actually. That assumes you're doing dividend reinvestment and such.

Vanguard's predictions of 3-4% over the next decade mean that if you do a straight dollar calculation you'd be closer to ~7%, because that 3-4% number includes inflation, once again.

It has to be said that this 7-8% average is inflated due to being at the end (?) of a ten year bull market.

Vangaurd is predicting low growth of 3-4% over the next decade.

7-8% is still nothing to scoff at. Better than a measly 1% in a savings account :D

There is a slight difference in outcome between 7 and 15 percent return though :).

There is no reputable ETF that will give you 15% return. Even 10% real return is incredibly unlikely.

I don't think 'unreputable' is fair - they're just a different (not 'low - medium'!) risk profile.

No, they're just non-existent. Anyone with a brokerage account had a ~20%+ return last year... it's over 30 years that matters.

I'm sure there's SOME ETF that managed to do 15% returns over a 30 yr period, but that's just selection bias after the fact. It doesn't mean that ETF would do well this year, or next.

What tickers?

Perhaps they're referring to leveraged ETFs like UPRO (Direxion Daily 3X Bull & Bear S&P) and TQQQ (ProShares UltraPro QQQ).

Be advised that they are not designed to be held over more than a day, although with the current bull market, they have not actually been hit by the decay factor inherent to all leveraged ETFs.

Other higher-risk ETFs would be things like solar/wind sector ETFs (e.g. TAN, FAN, PBW, IQCLN, QCLN, etc.) or bleeding-edge biotech (e.g. SBIO).