A study was designed to compare cardiac demands of snow shoveling to those of using an electric snow thrower. Ten subjects cleared tracts of snow using both methods and their maximum heart rates (beats per minute) were recorded. In this sample, the mean maximum heart rate during manual snow shoveling was 175 and the standard deviation was 15. Assuming the sample data have a bell-shaped distribution, you need to construct a 95% confidence interval estimate of the population mean for those who shovel snow. What value should you use for the standard error