Confidence Interval on Linear Combination of Means, Independent Groups (4 of 5)

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Finally, to compute t it is necessary to know the degrees of freedom. Since three values of s² went into MSE and each s² was based on n - 1 = 3 df, the degrees of freedom for MSE and for t are 3 x 3 =9. The value of t for the 95% confidence interval with 9 df can be found from a t table: It is 2.262. The lower limit (LL) of the confidence interval is:

LL = 1.5 - (2.262) (.6455) = 0.04.

The upper limit (UL) is:

UL = 1.5 + (2.262) (.6455) = 2.96.

The 95% confidence interval is therefore:



The results of this experiment show that, based on the 95% confidence interval, the average effect of aspirin and Tylenol is somewhere between 0.04 and 2.96 units more effective than a placebo.