TI-84 Plus CE field manual for IB Math
Confidence interval for μ
Use the t-interval routine to find a confidence interval for the population mean μ from summary statistics.
A sample of size n=20 has mean xˉ=115.3 and standard deviation sx=28. Find a 95% confidence interval for the population mean μ.
Press , move right to TESTS, and choose 8: TInterval.
Set Inpt to Stats (you have summary statistics, not raw data). Enter x̄: 115.3, Sx: 28, n: 20, and C-Level: 0.95.
Move down to Calculate and press .
The calculator's interval is (102.196…, 128.404…). Reported to 3 significant figures, the 95% confidence interval for μ is (102, 129).
Round the bounds outwards (lower bound down, upper bound up), so the reported interval still contains the calculator's exact one. Here 102.196… gives 102 either way, but 128.404… must round up to 129, not the 128 that ordinary 3 s.f. rounding would give.
Your turn
Work each one on your calculator, then check the answer.
- 1
A sample of n=15 has xˉ=120.7 and sx=42. Find a 90% confidence interval for μ.
- 2
A sample of n=28 has xˉ=280.8 and sx=45. Find a 99% confidence interval for μ.
- 3
A sample of n=22 has xˉ=210.1 and sx=26. Find a 95% confidence interval for μ.