The confidence interval for a series of findings can be calculated from the number of values, the mean, standard deviation and standard statistical tables.


Data assumptions: single mean, symmetrical distribution


confidence interval =

= (mean) +/- ((one-tailed value of Student's t distribution) * (standard deviation) / ((number of values) ^ (0.5))



• for a 95% confidence interval, the one-tailed value is for 2.5% (F 0.975, t 0.025)

• degrees of freedom = (number of values) - 1

• as the number of values increases, the closer the one-tailed value for t=0.025 approaches 1.96; at 120 degrees of freedom it is 1.98


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