# Mean and Standard Deviation on a TI-81

Here's how to compute some basic statistics on your TI-81:

### What you see

1) Press the [2nd][STAT] key. Note: To type [2nd][STAT], press the [2nd] key, then find STAT above the [MATRX] key, and press that key. CALC DRAW DATA
1:1-Var
2:LinReg
3:LnReg
4:ExpReg
5:PwrReg
2) Now press the key twice to highlight the word DATA on the top line of the screen. Note how the menu options change: CALC DRAW DATA
1:Edit
2:ClrStat
3:xSort
4:ySort
3) Press [2][ENTER] to select "ClrStat" and clear any previously entered data. ClrStat
Done
4) Press [2nd][STAT]again and to get back to the screen in step 2: CALC DRAW DATA
1:Edit
2:ClrStat
3:xSort
4:ySort
5) Now press [1] to select EDIT and begin entering data. DATA
x1=_
y1=1
4) Enter your list of numbers one at a time after the x's. A "1" will automatically appear after the y's.

For instance, to enter the list 1,3,5, type [1][ENTER][ENTER] [3][ENTER][ENTER] [5][ENTER][ENTER]
Try it, it will make more sense:

DATA
x1=1
y1=1
x2=3
y2=1
x3=5
y3=1
5) Now, to compute some statistics, press [2nd][STAT] to go to the statistics screen, [1] to select 1-Var statistics, and [ENTER] to compute those statistics.  1-Var
=3
x=9
x2=35
Sx=2
x=1.632993162
n=3
6) The =3 tells you the mean of your list is 4. The x=1.63299... tells you the [population] standard deviation of your list is 1.63299....

### Short summary:

Press [STAT] and select EDIT, ClrStat to clear out old data. Press [STAT] and select EDIT to enter your list of numbers. Remeber to enter your numbers on the "x" rows and leave the y's equal to 1. Press [STAT], select 1-Var Stats, and press [ENTER] to view the results.

### Test yourself:

Try to get the mean and standard deviation of   37.75, 38, 37, 38.5, and 37.5. You should get a mean of 37.75 and a standard deviation of 0.5.

## Bonus Topics:

### Frequency tables:

If you have many repeated data items, such as a frequency table:

 data item frequency 3 4 8 5 1 7 5 1

...meaning that your list contains four 3's, five 8's, etc., then you can enter the data items as x's and the corresponding frequencies as y's on the EDIT screen. For this list, you would enter

 DATA x1=3 y1=4 x2=8 y2=5 x3=1 y3=7

...etc.

### Help with medians:

Once you type a list of numbers in the list editor (see steps #1- #4 above), you can press [STAT] again, select DATA, then press [3][ENTER] to select xSort.Then when you edit the list again, you will find the list is displayed with the x-values in ascending order, making it easier to pick out the median.