Confidence interval with the right pivot

MediumStatistics~4m

From n=25n=25 daily P&L observations you get a sample mean of $50k and a sample standard deviation s=10ks=10\text{k}. The population standard deviation is unknown. Build a 95% confidence interval for the true mean daily P&L and report its half-width (in $k, to two decimals). Use t0.025, 24=2.064t_{0.025,\,24}=2.064.

Show hints (2)+
  1. σ\sigma unknown + small nn ⇒\Rightarrow use tn−1t_{n-1}, not the Normal 1.961.96.
  2. Half-width =t0.025,24⋅s/n=t_{0.025,24}\cdot s/\sqrt n.

Answer

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4.13 (± 0.02)

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