It is very difficult to account for the risk that the next market changing innovation will substantially alter the competitive landscape.

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One of the challenges with this style of study is that it is difficult to account for every aspect of life which might affect the probability of developing autism, such as family history.

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Regulators round the world sometimes find it difficult holding businesses to account for their infractions.

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His ability to do so makes it difficult for many people to accept that he cannot be held to account for them.

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"It's so difficult to take other factors into account - for example, moderate drinkers might have a better social environment, " he told BBC News Online.

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Unlike the hard-money ECB, most other leading central banks are happy to see their currencies depreciate which will make it all the more difficult for the euro area to run a current account surplus.

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