The head circumference is measured for a sample of 15 girls and a separate sample of 15 boys. What is the correct combination of degrees of freedom and the value of the multiplier t* for a 90% confidence interval for the difference in mean head circumference between girls and boys?
A) df = 13, t* = 1.77
B) df = 28, t* = 1.70
C) df = 28, t* = 2.05
D) df = 30, t* = 1.70

Question

Quizplus · Accepted Answer

The degrees of freedom (df) for two independent samples is calculated as the sum of their individual degrees of freedom, which is the sample sizes minus one for each. Therefore, df = (15-1) + (15-1) = 28. For a 90% confidence interval and 28 degrees of freedom, the t* value is approximately 1.70.

The Head Circumference Is Measured for a Sample of 15