A kind of Paired T Test, Student’s T distribution is used for finding confidence intervals for the population mean when the sample size is less than 30 and the population standard deviation is unknown. If you need to evaluate something with a population greater than 30, use the Z distribution => t distribution is flatter and wider than the z distribution. The t distribution becomes narrower (taller) as sample sizes increase, and gradually becomes very close to the Normal Distribution. Both z and t distributions are symmetric and bell-shaped, and both have a mean of zero.

*The more degrees of freedom, the better.

Student t-test would be used to compare two population means using samples from each. See this example.

Also used to test hypotheses about population means based on sample data.

## T Test Video

## Student’s T Distribution Example Questions

### Confidence of a mean 1

At a soda bottling factory, the normal filling specification (goal) is 16 fluid ounces. A sample of 20 bottles is tested with the following results: x = 16.13 fl oz and s = 0.24 fl oz. What interval would allow you to say, with 95% confidence, that the interval contains the actual mean of the filling process?

### Confidence of a mean 2

An automated computer attendant is supposed to respond to all inquiries within 15 seconds. The company claims with 99 percent confidence that the average response time is less than 17 seconds. A random sample of 25 calls reveals that X = 13.5 seconds and s = 0.95 seconds. Based on this data is the company correct?

### Confidence of a mean 3

A photography developing machine uses ink to produce photographs. A sample of 15 pictures showed an average of 1.5 mL of ink used per picture, with a standard deviation o 0.075 mL. Develop a 99% confidence band for the actual average amount of ink used by the machine to produce each photograph.

Also See:

http://stattrek.com/probability-distributions/t-distribution.aspx

## Comments (6)

Hi, I don´t get it. What is the formula t = (xbar – u) / (s/Sqrt(n)) useful for? In the examples all results are drawn with T-values from the table. Can you clarify?

Thanks 🙂

Julio, in the examples we are trying to solve the confidence interval. We get the critical t value from the chart. We then substitute that value into the t equation to find the confidence interval. If you look closely, you’ll see that the t = equation has been rewritten for u = and the critical t value is substituted in.

Hope that helps!

Hi I’m conducting a survey research on brand consumption. My indecent variable is (1) total $ spent per month on brand products and (2) total number of brands owned. (Chosen from a list of brands on questionnaire)

My dependent variables are (1) social media usage (2)belief on importance of brand names and (3) self esteem.

My hypothesis is that (1) increased social Media usage increases brand consumerism

(2) belief on importance of brand names increase brand consumerism

(3) lower self esteem increases brand consumerism

What type of tests do I need to run to test my hypothesis? I have one way anova but I’m not sure if that’s right

Hi Vic,

That’s an interesting set up. I’ve added this question to my membership site and will review as part of the upcoming paid black belt course.

I am getting ready to take the IASSC Lean Six Sigma Green Belt Certification Test. You should prominently include the IASSC Examination Reference Document and the testing rules with your study guide

Hi Scott,

Making a reference page would be helpful – not only for IASSC candidates, but for all other candidates.

From their site: IASSC provides a Reference Document in either paper or electronic form, subject to which form of exam one is taking. This document contains all formulas and tables one will need during the examination.

I can’t seem to find a version on-line. Do you know if they publish one for public consumption?