Telemarketers generally read from a prepared script when they make their sales calls. A firm decides to change this prepared script, making it both friendlier and shorter. Daily sales are recorded for a random sample of telemarketers, before and after the script change. The average difference-using a [(before the change) – (after the change)] order of subtraction-is + 4.2, with a sample size of 56. The differences have a standard deviation of 23.4. Do the data suggest that there is a difference in daily sales before and after the script change? Use ? = 0.05. What assumption do you have to make in order to answer this question?