You want to find out if the premiere of a new Marvel movie h…

You want to find out if the premiere of a new Marvel movie has an effect on the sales of Marvel comic books. When the newest movie was about to open, you went to seven comic book stores, and recorded their sales for Marvel comics. After the movie’s opening weekend, you went back to the same seven stores and recorded the sales of Marvel comics again. You recorded the sales in thousands of dollars in the table below. Calculate the mean of the differences. Round your answer to the third decimal place.