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Confidence Intervals: Confidence Interval, Single Population Mean, Standard Deviation Unknown, Student-T

Module by: Susan Dean, Barbara Illowsky, Ph.D.

Note: Your browser may not currently support MathML. See our browser support page for additional details. You can always view the correct math in the PDF version.

In practice, we rarely know the population standard deviation. In the past, when the sample size was large, this did not present a problem to statisticians. They used the sample standard deviation ss as an estimate for σσ and proceeded as before to calculate a confidence interval with close enough results. However, statisticians ran into problems when the sample size was small. A small sample size caused inaccuracies in the confidence interval. William S. Gossett of the Guinness brewery in Dublin, Ireland ran into this very problem. His experiments with hops and barley produced very few samples. Just replacing σσ with ss did not produce accurate results when he tried to calculate a confidence interval. He realized that he could not use a normal distribution for the calculation. This problem led him to "discover" what is called the Student-t distribution. The name comes from the fact that Gosset wrote under the pen name "Student."

Up until the mid 1990s, statisticians used the normal distribution approximation for large sample sizes and only used the Student-t distribution for sample sizes of at most 30. With the common use of graphing calculators and computers, the practice is to use the Student-t distribution whenever ss is used as an estimate for σσ.

If you draw a simple random sample of size nn from a population that has approximately a normal distribution with mean μμ and unknown population standard deviation σσ and calculate the t-score t = x¯ - μ ( s n ) t= x - μ ( s n ) , then the t-scores follow a Student-t distribution with n-1n-1 degrees of freedom. The t-score has the same interpretation as the z-score. It measures how far x¯x is from its mean μμ. For each sample size nn, there is a different Student-t distribution.

The degrees of freedom, n-1n-1, come from the sample standard deviation ss. In Chapter 2, we used nn deviations (x-x¯values)(x-xvalues) to calculate ss. Because the sum of the deviations is 0, we can find the last deviation once we know the other n-1n-1 deviations. The other n-1n-1 deviations can change or vary freely. We call the number n-1n-1 the degrees of freedom (df).

The following are some facts about the Student-t distribution:

  1. The graph for the Student-t distribution is similar to the normal curve.
  2. The Student-t distribution has more probability in its tails than the normal because the spread is somewhat greater than the normal.
  3. The underlying population of observations is normal with unknown population mean μμ and unknown population standard deviation σσ. In the real world, however, as long as the underlying population is large and bell-shaped, and the data are a simple random sample, practitioners often consider the assumptions met.

A Student-t table (See the Table of Contents 15. Tables) gives t-scores given the degrees of freedom and the right-tailed probability. The table is very limited. Calculators and computers can easily calculate any Student-t probabilities.

The notation for the Student-t distribution is (using T as the random variable)

TT ~ tdftdf where df=n-1df=n-1.

If the population standard deviation is not known, then the error bound for a population mean formula is:

EBM = t α 2 ⋅ ( s n ) EBM= t α 2 ⋅( s n ) t α 2 t α 2 is the t-score with area to the right equal to α 2 α 2 .

s s = the sample standard deviation

The mechanics for calculating the error bound and the confidence interval are the same as when σσ is known.

Example 1

Problem 1

Suppose you do a study of acupuncture to determine how effective it is in relieving pain. You measure sensory rates for 15 subjects with the results given below. Use the sample data to construct a 95% confidence interval for the mean sensory rate for the population (assumed normal) from which you took the data.

8.6; 9.4; 7.9; 6.8; 8.3; 7.3; 9.2; 9.6; 8.7; 11.4; 10.3; 5.4; 8.1; 5.5; 6.9

Note:

  • The first solution is step-by-step.
  • The second solution uses the TI-83+ and TI-84 calculators.

Solution A

To find the confidence interval, you need the sample mean, x¯x, and the EBM.

x¯ = 8.2267 s = 1.6722 n = 15 x =8.2267s=1.6722n=15

CL = 0.95 CL=0.95 so α = 1 - CL = 1 - 0.95 = 0.05 α=1-CL=1-0.95=0.05

EBM = t α 2 ⋅ ( s n ) EBM= t α 2 ⋅( s n )

α 2 = 0.025 t α 2 = t .025 = 2.14 α 2 =0.025 t α 2 = t .025 =2.14

(Student-t table with df=15-1=14df=15-1=14)

Therefore, EBM = 2.14 ⋅ ( 1.6722 15 ) = 0.924 EBM=2.14⋅( 1.6722 15 )=0.924

This gives x¯ - EBM = 8.2267 - 0.9240 = 7.3 x -EBM=8.2267-0.9240=7.3

and x¯ + EBM = 8.2267 + 0.9240 = 9.15 x +EBM=8.2267+0.9240=9.15

The 95% confidence interval is (7.30, 9.15).

You are 95% confident or sure that the true population average sensory rate is between 7.30 and 9.15.

Solution B

TI-83+ or TI-84: Use the function 8:TInterval in STAT TESTS. Once you are in TESTS, press 8:TInterval and arrow to Data. Press ENTER. Arrow down and enter the list name where you put the data for List, enter 1 for Freq, and enter .95 for C-level. Arrow down to Calculate and press ENTER. The confidence interval is (7.3006, 9.1527)

Glossary

Confidence Interval (CI):
An interval estimate for an unknown population parameter. This depends on:
  • The desired confidence level.
  • Information that is known about the distribution (for example, known standard deviation).
  • The sample and its size.
Degrees of Freedom (df):
The number of objects in a sample that are free to vary.
Error Bound for a Population Mean (EBM):
The margin of error. Depends on the confidence level, sample size, and the known or estimated population standard deviation.
Normal Distribution:
A continuous random variable (RV) with pdf f(x)=1σ2πe−(x−μ)2/2σ2f(x)=1σ2πe−(x−μ)2/2σ2 size 12{ ital "pdf"= { {1} over {σ sqrt {2π} } } e rSup { size 8{ - \( x - μ \) rSup { size 6{2} } /2σ rSup { size 6{2} } } } } {}, where μμ is the mean of the distribution and σσ is its standard deviation. Notation: XX ~ N μ σ N μ σ . If μ=0μ=0 and σ=1σ=1, the RV is called the standard normal distribution.
Standard Deviation:
A number that is equal to the square root of the variance and measures how far data values are from their mean. Notation: ss for sample standard deviation and σσ for population standard deviation.
Student-t Distribution:
Investigated and reported by William S. Gossett in 1908 and published under the pseudonym Student. The major characteristics of the random variable (RV) are:
  • It is continuous and assumes any real values.
  • The pdf is symmetrical about its mean of zero. However, it is more spread out and flatter at the apex than the normal distribution.
  • It approaches the standard normal distribution as n gets larger.
  • There is a "family" of t distributions: every representative of the family is completely defined by the number of degrees of freedom which is one less than the number of data.

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