TI-Nspire CX II 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.
From the Scratchpad calculator, press → 6: Statistics → 6: Confidence Intervals → 2: t Interval. Set Data Input Method to Stats.
Fill in the dialog, moving between fields with : x̄: 115.3, sx: 28, n: 20, and C Level: 0.95.
Select OK and press . The interval lands on the calculator page.
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. Take the lower bound down and the upper bound up, so your reported interval still contains the calculator's exact one. Never round a bound in the direction that would narrow the interval. Here the lower bound 102.196… gives 102 either way, but the upper bound 128.404… must round up to 129, not the 128 that ordinary 3 s.f. rounding would give, so that 128.404 still lies inside the interval.
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.