An atomic absorption method for the determination of the amount of iron in used jet engine oil was found, from pooling 30 triplicates analyses, to have a standard deviation s = 2.4 g Fe/mL. If s is a good estimate of , the calculated result based on the mean of four analyses at 95% confidence interval was 18.5 +/- 2.4 g Fe/mL. How many replicate measurements are needed to decrease the 95% confidence interval for this analysis to +/-1.4 g Fe/mL.