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