Mr Greenspan argues that it is hard to be sure what constitutes a bubble.
"It's hard to be sure, " he said, because the detailed inner workings of the Sun were a mystery.
It is hard to be sure whether this debate heralds a lasting shift in Canada's openness to immigration.
ECONOMIST: Immigration in Canada: A smaller welcome mat | The
In that atmosphere, it was hard to be sure some of the most basic research was undertaken when it should have been.
Most of the regulatory ideas are still expressed in somewhat general terms, so it is hard to be sure if they are reasonable or over-the-top.
Jonathan Hopkin of the London School of Economics says that Britain's good reputation is not entirely misplaced, but the lack of reliable measures makes it hard to be sure.
The shares of many technology companies may now look cheap, but it will be hard to be sure until their managers express even a small degree of confidence that the profit outlook has stopped worsening.
Even if it is hard to be sure how big these effects are, the size of the very top level seems to matter, so Britain's 50% rate is more dangerous than Mr Obama's proposal to raise America's top federal income-tax rate from 35% to 39.6%.
Worryingly, they also appear not to have been audited by specialists in line with the highest standards for voluntary credits, says John Romankiewicz at NEF: it is hard to be sure, for example, that people really rode their bikes to work during the Olympics rather than jumping in taxis.
But we are also going to be pushing hard to make sure that you are not engaging in gaming the system.
Swann, famed for his knack of first-over wickets, was equally unable to part North and Robinson as England received an early lesson about the patience and hard work sure to be required this winter.
Brecht could hardly have imagined such a document: a capitalist enterprise that feeds on the misery of man trying as hard as it can to be sure that nothing is done to decrease that misery.
And the complexity of campaign-finance law makes it hard even for well-meaning candidates to be sure they are not breaking it.
If it comes into law, we will have to work very hard to make sure that families and children remain important and biological families will have to be shored up in other ways.
To be sure, I was hard-wired to love the NFL. My father, an early Burger King franchisee, knew one of the Miami Dolphins owners, and I was rarely photographed as a child without my No. 12 Bob Griese jersey.
Just be sure to remove all the hard parts, such as wings and legs, before you use the adults.
For sure, you will be hard pressed to restore your nest egg by owning bonds.
The opponents include the prominent Swiss-based (but Russia-linked) Trans World Group, which says that a ban on tolling is impractical, because Russian firms cannot be sure of always having the hard currency to pay for the continuous supply of imported ore they need.
It's always hard to pick a winner, and I'm sure it'll be harder than ever this time around.
This is how hard it is: Republican leaders cannot be sure of getting enough Republicans to vote for their own plan to cut the debt and raise borrowing.
It said while the needs of the sickest patients were given priority, it was working hard to make sure that all patients get the care they needed while waiting to be admitted.
While it's hard to know for sure, we don't expect to see anything that'll be exceptionally innovative on a widespread scale -- mostly.
Some roads will be affected, including temporary closures, but officials said they were working hard to make sure that people could "go about their daily business as usual".
"She, I'm sure, is going to be a very diligent senator, work hard, get committee assignments where she has a real interest, and will be a very important part of this body just like every other senator is, " said Sen.
"Ending the exemptions from age classification for hard copy music videos will be an important step forward in making sure that children are not inadvertently exposed to unsuitable material, " said Mr Bailey.
Die-hard Pittsburgh and Green Bay fans take note: when you don your team jerseys this weekend, be sure to keep your emotions under control.
Then the hard work begins, and that is the work I want to be a part of in the months ahead: Making sure this doesn't happen again.
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