What is the t-distribution?

The t-distribution explains the standardized distances of sample means to the population mean as soon as the population standard deviation is not known, and the monitorings come indigenous a normally spread population.

You are watching: Explain why the t-distribution has less spread as the number of degrees of freedom increases.

Is the t-distribution the same as the Student’s t-distribution?


What’s the an essential difference between the t- and z-distributions?

The standard normal or z-distribution assumes the you understand the population standard deviation. The t-distribution is based on the sample traditional deviation.

t-Distribution vs. Normal distribution

The t-distribution is comparable to a regular distribution. It has a precise mathematical definition. Instead of diving into complex math, stop look in ~ the advantageous properties the the t-distribution and also why that is vital in analyses.

Like the common distribution, the t-distribution has a smooth shape.Like the normal distribution, the t-distribution is symmetric. If friend think around folding that in half at the mean, each side will certainly be the same.Like a conventional normal distribution (or z-distribution), the t-distribution has a typical of zero.The normal circulation assumes the the population standard deviation is known. The t-distribution does not make this assumption.The t-distribution is characterized by the degrees the freedom. This are concerned the sample size.The t-distribution is most advantageous for little sample sizes, when the populace standard deviation is not known, or both.As the sample size increases, the t-distribution becomes more similar to a regular distribution.

Consider the adhering to graph comparing three t-distributions through a standard normal distribution:


Tails for hypotheses tests and the t-distribution

When you execute a t-test, you inspect if your test statistic is a an ext extreme value than expected from the t-distribution.

For a two-tailed test, you look at both tails the the distribution. Number 3 listed below shows the decision procedure for a two-tailed test. The curve is a t-distribution through 21 levels of freedom. The value from the t-distribution with α = 0.05/2 = 0.025 is 2.080. Because that a two-tailed test, you refuse the null theory if the check statistic is larger than the absolute value of the recommendation value. If the check statistic value is one of two people in the lower tail or in the upper tail, you disapprove the null hypothesis. If the check statistic is in ~ the two reference lines, then you fail to reject the null hypothesis.


How to use a t-table

Most world use software to perform the calculations necessary for t-tests. But many statistics books still present t-tables, for this reason understanding just how to use a table might be helpful. The steps listed below describe how to usage a common t-table.

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Identify if the table is because that two-tailed or one-tailed tests. Then, decide if you have a one-tailed or a two-tailed test. The columns for a t-table identify different alpha levels.If you have a table because that a one-tailed test, you have the right to still usage it for a two-tailed test. If you collection α = 0.05 for her two-tailed test and also have just a one-tailed table, then usage the obelisk for α = 0.025.Identify the degrees of freedom for your data. The rows the a t-table exchange mail to different levels of freedom. Many tables go as much as 30 degrees of freedom and then stop. The tables assume world will use a z-distribution for bigger sample sizes.Find the cabinet in the table at the intersection of your α level and also degrees that freedom. This is the t-distribution value. Compare her statistic to the t-distribution value and also make the proper conclusion.