The TInterval Command
TINTERVAL.GIF

Command Summary

Using either already-calculated statistics, or a data set, computes a t confidence interval.

Command Syntax

TInterval [list, [frequency]], [confidence level]
(data list input)

TInterval mean, std. deviation, sample size, [confidence level]
(summary stats input)

Menu Location

When editing a program, press:

  1. STAT to access the statistics menu
  2. LEFT to access the TESTS submenu
  3. 8 to select TInterval, or use arrows

(this key sequence will give you the TInterval… screen outside a program)

Calculator Compatibility

TI-83/84/+/SE

Token Size

2 bytes

The TInterval command calculates a confidence interval for the mean value of a population, at a specific confidence level: for example, if the confidence level is 95%, you are 95% certain that the mean lies within the interval you get. Use TInterval when you have a single variable to analyze, and don't know the standard deviation. The TInterval assumes that your distribution is normal, but it will work for other distributions if the sample size is large enough.

There are two ways to call the TInterval command: by supplying it with needed sample statistics (mean, sample standard deviation, and sample size), or by entering a list and letting the calculator work the statistics out.

Sample Problem

You want to know the average height of a student at your school. You haven't asked everyone, but you took a random sample of 30 people and found out their heights (and stored it to L1). You've decided to use a 95% confidence interval.

Since the syntax for entering a data list is TInterval list, confidence level, here is your code:

:TInterval L1,95
you can also use
:TInterval L1,.95

Alternatively, you could calculate the mean, sample size, and standard deviation, and enter those instead. The sample size in this case is 30; let's say the mean was 63 inches and the standard deviation 6.2 inches.

The syntax for entering statistics is TInterval mean, std. deviation, sample size, confidence level, so your code would look like:

:TInterval 63,6.2,30,95
you can also use
:TInterval 63,6.2,30,.95

Of course, the main use of the TInterval command is in a program. While you can enter the TInterval command on the home screen as well (just look in the catalog for it), it would probably be easier to select TInterval… from the STAT>TEST menu (see the sidebar).

Advanced Uses

As with most other statistical commands, you can enter a second list after the data list, to add frequencies (only with the data list syntax, of course). The frequency list must contain non-negative integers, and can't be all 0.

Optimization

Using the data list syntax, all the arguments are optional: the calculator will assume you want to use L1 for your data unless another list is supplied, and that the confidence level you want is 95% unless you give another one. Using the summary stats syntax, the confidence level is also optional - again, the calculator will assume 95%. This means we can rewrite our code above in a simpler manner:

:TInterval L1,95
can be just
:TInterval
:TInterval 63,6.2,30,95
can be
:TInterval 63,6.2,30

Error Conditions

  • ERR:DATA TYPE occurs if complex numbers are used (in some cases, ERR:ARGUMENT is thrown instead).
  • ERR:DIM MISMATCH occurs if the data and frequency lists aren't the same size.
  • ERR:DOMAIN occurs in any of the following cases:
    • The confidence level isn't in the range (0 .. 100).
    • The standard deviation isn't positive.
    • The sample size isn't an integer greater than 1.
  • ERR:STAT occurs if the frequency list's elements aren't integers.

Related Commands

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