Suggested answer to exercise: Confidence intervals for mean FEV1, 1

Question 1: Using SPSS, calculate a 95% confidence interval for mean FEV1.

Suggested answer

To do this we choose Analyze, Descriptive Statistics, Explore, variable FEV1, OK.

We get the following output:

Descriptives
			Statistic	Std. Error
FEV1 (litre)	Mean		4.0607	.08895
	95% Confidence Interval for Mean	Lower Bound	3.8825
		Upper Bound	4.2389
	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

SPSS also gives other output, a stem and leaf plot and a box and whisker plot.

The 95% confidence interval is 3.8825 to 4.2389 litres.

To two decimal places, this is 3.88 to 4.24 litres.

As we know the units, litres, we should state them.

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Last updated: 31 January, 2012.

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