But, she conceded, "it's difficult to know the exact number".
"It is difficult to know the Iraqi detainee population.
It is very difficult to know the answer.
It's difficult to know the history of anyone.
In the print article we stress that it is difficult to know the level of commitment that savers want.
"Around here it's very difficult to know the future," said Seamus Clarke, the pub's 45-year-old owner.
Most of these apps and Web sites are so intertwined that it's difficult to know the difference.
It is not difficult to know the difference between someone who is consenting to penetration and someone who isn't.
It is difficult to know the exact number of suits, but I-Flow reported in November that it was a defendant in 191 chondrolysis cases involving 412 patients.
It is difficult to know the exact number of Catholics in China, but conservative estimates by Fides, the Vatican news agency, range from 8 to 12 million.
The remaining £24.5bn were for electricity and gas utilities, networks, financiers and others, where it is difficult to know the level of fossil fuel investment.
