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6-6 Confi dence Interval
A confi dence interval is a range (interval) that includes a statistical value, usually the
population mean.
A confi dence interval that is too broad makes it diffi cult to get an idea of where the population
value (true value) is located. A narrow confi dence interval, on the other hand, limits the
population value and makes it diffi cult to obtain reliable results. The most commonly used
confi dence levels are 95% and 99%. Raising the confi dence level broadens the confi dence
interval, while lowering the confi dence level narrows the confi dence level, but it also
increases the chance of accidently overlooking the population value. With a 95% confi dence
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, confi dence interval width, and confi dence level. The confi dence
level changes in accordance with the application.
1-Sample
Z Interval calculates the confi dence interval for an unknown population mean
when the population standard deviation is known.
2-Sample
Z Interval calculates the confi dence interval for the difference between two
population means when the population standard deviations of two samples are known.
1-Prop
Z Interval calculates the confi dence interval for an unknown proportion of successes.
2-Prop
Z Interval calculates the confi dence interval for the difference between the propotion
of successes in two populations.
1-Sample
t Interval calculates the confi dence interval for an unknown population mean
when the population standard deviation is unknown.
2-Sample
t Interval calculates the confi dence 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 confi dence 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 confi dence interval
functions.
6-6-1
Confi dence Interval
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