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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.