Shares of Smithfield rose $ 1.64 yesterday to close at $ 20.85 , a rise of 8.5 per cent , on the New York Stock Exchange .
Shares of Smithfield closed up $ 1.64 , or 8.5 percent , at $ 20.85 , on the New York Stock Exchange .
most convincing evidence because you are measuring the effects of an intervention between two different groups.