6. Confidence Interval

Important!

• Confidence interval calculations cannot be performed on the fx-7400GII.

A confidence interval is a range (interval) that includes a statistical value, usually the population mean.

A confidence interval that is too broad makes it difficult to get an idea of where the population value (true value) is located. A narrow confidence interval, on the other hand, limits the population value and makes it difficult to obtain reliable results. The most commonly used confidence levels are 95% and 99%. Raising the confidence level broadens the confidence interval, while lowering the confidence level narrows the confidence level, but it also increases the chance of accidently overlooking the population value. With a 95% confidence interval, for example, the population value is not included within the resulting intervals 5% of the time.

When you plan to conduct a survey and then t test and Z test the data, you must also consider the sample size, confidence interval width, and confidence level. The confidence level changes in accordance with the application.

1-SampleZ Interval calculates the confidence interval for an unknown population mean when the population standard deviation is known.

2-SampleZ Interval calculates the confidence interval for the difference between two population means when the population standard deviations of two samples are known.

1-PropZ Interval calculates the confidence interval for an unknown proportion of successes.

2-PropZ Interval calculates the confidence interval for the difference between the proportion of successes in two populations.

1-Samplet Interval calculates the confidence interval for an unknown population mean when the population standard deviation is unknown.

2-Samplet Interval calculates the confidence interval for the difference between two population means when both population standard deviations are unknown.

On the initial STAT mode screen, press (INTR) to display the confidence interval menu, which contains the following items.

•(INTR)(Z) ... Z intervals (page 6-36)

(t) ... t intervals (page 6-37)

After setting all the parameters, use A to move the highlighting to “Execute” and then press the function key shown below to perform the calculation.

•(CALC) ... Performs the calculation.

•There is no graphing for confidence interval functions.

SGeneral Confidence Interval Precaution

Inputting a value in the range of 0 C-Level < 1 for the C-Level setting sets a value you input. Inputting a value in the range of 1 C-Level < 100 sets a value equivalent to your input divided

by 100.

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