Piranhas were dumped into the lake for years, but they can't establish, because the temperature isn't right for them.
This relationship known as a "cost function"—can be tricky to establish because firms often have multiple inputs and outputs.
Independent death tolls are difficult to establish because both sides regularly distort the figures to their benefit, and the war zones are restricted by the military.
Their contribution to reduction in crime is difficult to establish because other factors might be at work, including changes in police strategies.
Today, Southampton detectives said that the investigation was complicated by Ms. Grubman's departure from the scene, and that details about the injuries were difficult to establish because the victims were taken to four different hospitals.
How many exorcisms Amorth performed over the next 14 years is hard to establish, because the number seemed to climb after his retirement until, in 2013, it reached the improbable tally of 160,000.
In France, Mr. Vivet said, there is another such law, on "moral harassment," which is "very difficult to implement and hard to establish, because we need elements of proof".
This seems to be an easy element to establish because the wild gyrations in the market on the first trading day after the downgrade shows how investors viewed it.
Most of the local government suits -- some of which have already been dismissed -- have claimed negligence by gun makers, which can be difficult to establish because it requires a showing that the defendants had a legal duty regarding the eventual use of the guns and that they violated that duty.
However, other elements of the banking union are proving harder to establish because of opposition by some governments.
Equation 1 is possible to establish because the correlation between electron and hole positions, induced by the Coulomb interaction, is not enough strong.
