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Another variance and sample size question

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Another variance and sample size question

Hi, I know that the variance halves when the sample size doubles and this makes sense to me when using the variance to compute a confidence interval for the mean. But I presume that relationship doesn’t mean that you can adjust data with it?

We measure combustions stability in an engine with stdev(IMEP) where IMEP is   the average pressure in the cylinder. We normally do stdev(IMEP) over 300 cycles (n = 300). My colleague collected a lot of data with only 100 cycles and has asked me if this can be converted to 300 cycle equivalent. I don’t think so but not completely sure about this so would like to ask the maths community?

Any help much appreciated.

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