Shares of Smithfield closed up $ 1.64 , or 8.5 percent , at $ 20.85 , on the New York Stock Exchange .
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 .
learning by 'trying' a response and being 'punished' or 'rewarded' depending on whether the response was the desired one.