Suggested answer to exercise: Confidence intervals for mean FEV1, 2

Question 2: Calculate a 99% confidence interval for mean FEV1.

Suggested answer

To do this we choose Analyze, Descriptive Statistics, Explore, variable FEV1, as before. We then click Statistics and change the Confidence Interval for Mean from 95 to 99, Continue, OK.

We get the following output:

Descriptives
			Statistic	Std. Error
FEV1 (litre)	Mean		4.0607	.08895
	99% Confidence Interval for Mean	Lower Bound	3.8235
		Upper Bound	4.2979
	5% Trimmed Mean		4.0561
	Median		4.1000
	Variance		.451
	Std. Deviation		.67153
	Minimum		2.85
	Maximum		5.43
	Range		2.58
	Interquartile Range		.99
	Skewness		.064	.316
	Kurtosis		-.857	.623

The 99% confidence interval is 3.8235 to 4.2979 litres.

To two decimal places, this is 3.82 to 4.30 litres.

Note that rounding up the 9 would make this 10, so the preceding decimal place goes up by one and the 9 becomes 0.

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Last updated: 6 October, 2006.

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