Aresearcher wishes to determine the difference in two population means. to do this, she randomly samples 9 items from each population and computes a 90% confidence interval. the sample from the first population produces a mean of 780 with a standard deviation of 240. the sample from the second population produces a mean of 890 with a standard deviation of 280. assume that the values are normally distributed in each population. the point estimate for the difference in the means of these two populations is