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6-6 Confidence Interval
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)1(Z) ... Z intervals (page 6-6-3)
2(t) ... t intervals (page 6-6-8)
# There is no graphing for confidence interval
functions.
6-6-1
Confidence Interval