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20010101
1-3 Confidence Interval (INTR)
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-Sample Z Interval calculates the confidence interval for an unknown population mean
when the population standard deviation is known.
2-Sample Z Interval calculates the confidence interval for the difference between two
population means when the population standard deviations of two samples are known.
1-Prop Z Interval calculates the confidence interval for an unknown proportion of
successes.
2-Prop Z Interval calculates the confidence interval for the difference between the propotion
of successes in two populations.
1-Sample t Interval calculates the confidence interval for an unknown population mean
when the population standard deviation is unknown.
2-Sample t 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 4 (INTR) to display the confidence interval menu,
which contains the following items.
• 4(INTR)b(Z) ... Z intervals (p. 1-3-3)
c(T) ... t intervals (p. 1-3-8)
# There is no graphing for confidence interval
functions.
1-3-1
Confidence Interval (INTR)
20011101