And cancer also can sit latent for years, making it hard to link cause and effect.
Not taking any chances, the Bush administration also worked hard to link Saddam to Hitler in the public mind.
But it isn't hard to link the decision with the fact that Murdoch is in London this week.
He worked hard to link Mr. Labastida to Carlos Salinas de Gortari, the disgraced former president who is now in forced exile in Ireland.
India was among the developing countries that cried foul when the administration pushed hard to link new trade talks with labor rights, a position many developing countries viewed as a disguised effort to protect workers in wealthy countries from competition.
But it is hard to link pollution to specific sources, said Philip M. Fine, manager of atmospheric measurements for the South Coast Air Quality Management District, the air pollution control agency for all or parts of four Southern California counties.
In the future, big old things may need to survive climate change, which some experts fear will produce more extreme weather, including droughts or deluges (although it may be hard to link climate change to any specific event, like the deluge in New York last week).
It becomes hard to link the gruff character played by Mr Spall with the spellbinding pictures that hang in Tate Britain today.Consequently, when Mr Leigh (interviewed here) does try to show Turner as a high aesthete, the result feels contrived, such as when Turner defends Claude Lorrain, a French painter, from the attacks of a young (and highly pompous) John Ruskin.
Until now, it was surprisingly hard to link incoming phone calls to ads.
Because DNA methylation is tissue-specific, having a mixed cell population makes it hard to link the observed DNA methylation patterns to the disease rather than to changes in cell populations.
