Help with 2-sample T Interval

#1
Hi. I have been trying this problem for awhile and I am getting so frustrated. I have been able to complete every problem but this one.

"The final prices from an online auction for 2 sales methods are shown below. Construct a 95% confidence interval for comparing the 2 pop means.

1.buy it now option- 220, 200, 225, 240, 225, 210, 225
2.bidding option- 255, 262, 225, 248, 265, 231, 228, 258, 248, 263, 204, 211, 226, 220, 225, 230, 214, 229


We are supposed to use our calculators to do the problem and I must be doing something wrong. I am using 2 sample t interval and keep getting (-28.91, -.99) when the answer is ( -31.13, 1.21)

What am I entering wrong? thank you
 

TheEcologist

Global Moderator
#2
Hi. I have been trying this problem for awhile and I am getting so frustrated. I have been able to complete every problem but this one.

"The final prices from an online auction for 2 sales methods are shown below. Construct a 95% confidence interval for comparing the 2 pop means.

1.buy it now option- 220, 200, 225, 240, 225, 210, 225
2.bidding option- 255, 262, 225, 248, 265, 231, 228, 258, 248, 263, 204, 211, 226, 220, 225, 230, 214, 229


We are supposed to use our calculators to do the problem and I must be doing something wrong. I am using 2 sample t interval and keep getting (-28.91, -.99) when the answer is ( -31.13, 1.21)

What am I entering wrong? thank you

Your answer is correct for an alpha of 0.05 (95% CI, critical T [df:16.7]= +/- 2.11). When assuming the students T-distribution, non equal variances and two tailed integration of the p-values. Therefore if the question was to calculate it in this way you did it correctly.

The answer given is the p-value with the same conditions only with an alpha of 0.025 (97.5% CI, critical T[df:16.7] = +/- 2.448).

I believe your calculations are correct, and the given answer wrong.

Hope this helps