In 1988, a study followed 89 sedentary men for a year. 42 men were on a diet, and the remaining 47 were on an exercise routine. The group on the diet lost 7.2 kg on average, with a standard deviation of 3.9 kg. Construct a 99.7% confidence interval for the true average number of kg lost on diet.
From what we understand: We want to use the SD to find the variance and then use that to find the sample mean, but when we do that we got impossible numbers, so we couldn't identify p or n.
Does anyone know how we should start this?
You could do that, but I'm not sure it's helpful here....
Not quite. Note that you're actually given the sample mean explicitly as a part of the problem text.
I'm not sure what "$p$" you hope to identify, but this isn't a problem about a proportion; it's a problem about developing a confidence interval for a mean. Your task is to find the way that this has been done in your class (almost certainly either (a) with a certain table of numbers and a formula, or (b) with statistical software) and then to replicate that process.