7-10-1

Confidence Intervals

7-10 Confidence Intervals

A confidence interval is a range of values that has a specified probability of containing the parameter being estimated.

A confidence interval that is too broad makes it difficult to get an idea of where the parameter (actual value) is located. A narrow confidence interval, on the other hand, limits the parameter range and makes it possible to obtain highly accurate results.

The commonly used confidence levels are 68%, 95% and 99%. Raising the confidence level broadens the confidence interval. Conversely, lowering the confidence level narrows the confidence interval, but it also creates the risk that parameters will be missed. With a confidence interval of 95%, for example, there is a 5% probability that a parameter will not be within the interval.

The following is a list of confidence intervals and a description of what each obtains.

Confidence Interval Name

Description

 

 

1-Sample Z Interval

Obtains the confidence interval for the population mean when the

population standard deviation is known.

 

 

 

 

Obtains the confidence interval for the difference between population

2-Sample Z Interval

means when the population standard deviations of two populations are

 

known.

 

 

1-Prop Z Interval

Obtains the confidence interval of the proportion of successes in a

population.

 

 

 

2-Prop Z Interval

Obtains the confidence interval of the difference between the

proportions of successes of two populations.

 

 

 

1-Sample t Interval

Obtains the confidence interval for the population mean when the

population standard deviation is unknown.

 

 

 

2-Sample t Interval

Obtains the confidence interval for the difference between two

population means when the population standard deviations are

 

unknown.

 

 

kGeneral Confidence Interval Precautions

If you input a C-Level (confidence level) value in the range of 0 <C-Level < 1, the value you input is used. To specify a C-Level of 95%, for example, input “0.95”.

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