Even if we, say, halve all carbon emissions today, it would not have an immediate effect on atmospheric CO2 concentrations.
If we can delay the onset of dementia by five years, we can halve the number of people who will die from the disease.
The government is to use about a third of the surplus to fund a series of minor tax cuts aimed largely at Hong Kong's low and middle-income earners, including a populist move to halve the duty of wine and beer.
One would halve the number of elected representatives to departmental and regional assemblies.
A 12-year study on Japanese women found that drinking 3 or more cups of coffee per day may actually halve the risk of developing colon cancer.
Although the TB burden may be falling globally, the decline is not fast enough to meet the impact targets set by the Stop TB Partnership - to halve the 1990 prevalence and death rates by 2015.
Trials have shown it can halve the number of people who return to smoking after trying to give up.
Mr Obama vows to halve it within four years, but that will still leave it bigger than the deficits for which he once lashed Mr Bush.
Together these impacts would halve the costs of dealing with climate change by 2050.
Breastfeeding could more than halve the risk of metabolic syndrome, a mix of symptoms including high blood pressure, obesity and cholesterol that if untreated can lead to major illness.
If fully implemented, the Global Plan will treat 50 million people for TB, halve TB prevalence and death rates and save 14 million lives.
The team predicts that further increases in carbon dioxide will halve the amount of water lost to the atmosphere.
Schools have been hit by the loss of teachers to tourism jobs (and by a decision to halve class sizes to 15).
Greece may have to halve its debt burden.
This number can halve later in life.
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