'sharpest increase' is correct and usable in written English.
You can use it to describe a change in a quantity or measure that has gone up very quickly or significantly. For example, "The sharpest increase was recorded in the number of hours worked each week".
That was the sharpest increase since February.
The sharpest increase in the budget over last year was a 10percentt rise in financing for interdiction.
The price of wholesale gasoline alone rose 9.3percentt last month, the sharpest increase of any product.
More than a million jobs have been added to total nonfarm payrolls over the past four months, the sharpest increase since early 2000.
The number of primary school pupils is projected to increase by more than 500,000 between 2010 and 2018, with the sharpest increase in London.
Among all funds, sales of international funds showed the sharpest increase this year, rising to an estimated $7.6 billion, from $1.9 billion in 1985.
The sharpest increase in energy prices in the United States this winter has occurred for heating oil in the New York area and in New England.
The political scientists reported the sharpest increase in improvement between 1950 and 1975, a period that included the civil rights movement and the Voting Rights Act.
A report published last week shows the sharpest increase in the US poverty rate since 1994, with one in seven now living in poverty.
Food prices are soaring, and the government said on Friday that the consumer price index in March had risen 5.4 percent, its sharpest increase in nearly three years.
The sharpest increase coincides with the start of the economic downturn and the rise in house prices, which is preventing first-time buyers from entering the property market.
