Precise economist forecasts need to be treated with a grain of salt and widespread support for a particular investment doesn't always indicate a sure thing, an investment strategist warns.

AMP Capital Investors head of investment strategy Dr Shane Oliver said the most important thing investors need to be aware of is that crowd support for a particular investment doesn't always translate as a winning decision.

"People tend to get comfort out of what they're friends are doing, or what's popular, or by media messages," Dr Oliver said.

"So they tend to have that crowd support and the trouble is ... you find out you're getting in at the top and it's often the time you should be getting out, not getting in.

"I mean, there's nothing wrong with being where the crowd is, sometimes it can take you in the right direction, but you've got to be conscious of that. There's a danger of going like a lemming off a cliff."

Dr Oliver, who is also the AMP Capital Investors chief economist, said forecasts needed to be treated with care because economists don't always adjust their outlook as new information becomes available.

"Like everyone, market forecasters suffer from numerous psychological biases and precise forecasts are conditional upon information when the forecast is made, but need adjustment as new facts come to light," he said.

"You have to change with those facts.

"Forecasts for economic and investment indicators are useful, but need to be treated with care."

In a research note called `Myths and Mistakes Investors Often Make', Dr Oliver outlines the myths that catch out investors.

He also warns that economists don't have a perfect crystal ball for seeing the economic future.

"If I'm asked for the All Ords at the end of the year or the cash rate at the end of the year, it's a nice neat summary way of saying this is my view," Dr Oliver told AAP.

"But I know, and the people who put the surveys together know, a lot of things are going to change between now and then.

"When you look at consensus forecast and compare it to the actual outcome, you'll often find they're wide of the mark."

Dr Oliver said poor economic growth often provided fertile ground for market expansion in the short term.

"This is true over the long term and at various points in the economic cycle," he said in the note.

He said that while the gross domestic products (GDP) of some of the world's largest economies contracted in 2009, stock markets soared.

American GDP declined 5.7 per cent in the first quarter of 2009, according to data from the US Commerce Department, while the Dow Jones industrial average climbed from a 12-year low of 6547.5 points on March 3 to reach 8440.87 on June 2.

Since March 6, the Australian share market climbed 898 points to a five-month high of 4009 points on Thursday, while Australia recorded 0.4 per cent growth for the March quarter, narrowly avoiding the definition of a technical recession.