Hi, I am an 3rd engineering student working on my coop report. My job was to design routes for survey work, and come up with figures of potential time savings if this was to be done on a grand scale. The sites are divided into 15 regions (by geography) but are not in any particular order. From a population of 2062 survey sites, I chose a sample of 155 (1 of the regions). I drove these unplanned, took X number of minutes, then I drove it planned it took Y number of minutes. My time savings was 8.6%, and I am looking to turn this into some sort of confidence interval. To do so I was going to break it down as time per read site (min/read) and thus my total time in minutes / 155 reads would be used as my mean for each situation. Then I was going to compare them as a difference between means, this is where I would have the confidence interval at 95%. Problem is I do not know how I can find a standard deviation, as I have not actually mesured these time per read on an individual basis, and doing so would be an impossible task. Am a taking a wrong approach in how I am going to use the data or what statistical approach I am using? Is there a way I can still get some sort of standard deviation from the sample? Thanks for the help. John.