WHAT IS THE MEAN DEVIATION FOR 121 122 123 124 125 126?

Mean absolute deviation.
Let's break it down into its components.
Mean - Average
Absolute - absolute value - sign doesn't matter - if negative, make positive
Deviation - difference from the mean

Let's do an example. Population 3 7 4 2. Total 16. Mean (Average) 16/4 = 4.

Deviations
3 - 4 = -1 absolute value 1
7 - 4 = 3
4 - 4 = 0
2 - 4 = -2 absolute value 2

Sum of these deviations 1 + 3 + 0 + 2 = 6
Mean (average) of these deviations is 6 / 4 = 1.5.

Good luck,

Paul

The mean is the arithmetic average, so add up all the numbers and divide by the number of numbers.

The standard deviation is a little more involved. The formula and example are explained here:
Standard Deviation Formulas


Good luck.

Paul

The standard deviation is a measure of how "tight" the samples are distributed around your mean.
In layman's terms, a small standard deviation indicates that most of your measurements are in the vicinity of the means; a large standard deviation corresponds to readings that are all over the place.
You could also say that the smaller the SD, the more your mean is representative of the data set.
For a better explanation, just look up Standard deviation on Wikipedia!

Population size:4
Mean (?): 23.45
Standard deviation (?): 4.5986411036305

If you mean the standard deviation, there are two shifted keys on the third row marked with a lower-case Greek sigma. sigma_xn is the population standard deviation and sigma_xn-1 is the sample standard deviation.

As the name says, it's the mean of the absolute deviations. See http://en.wikipedia.org/wiki/Absolute_deviation

Here is one technique I used on the TI84Plus.
Create two lists list1(50) and list2(50) of equal dimension (same length)
You will have to find how to do it on ClassPad.
Fill list2 with zeros
Fill list1 with your data.
Sort the list in ascending order (if it is not sorted)
Do standard deviation calculation on list1 (or calculate the average)
The mean of the data is in a special variable (xbar)
From the command line using the list syntax calculate abs(list1-xbar) and store results in list2, overwiting the zeros.
Perform standard deviation on the new list2. You will get the mean deviation from the mean in the variable xbar.

A simple request from you. Can you please not use MINUS as a verb? By doing so you are propagating incorrect usage. There is a short and correct way to express the operation: to add.

64 62 52 50 70 60 55 20 25 48 35 55 55 47 64 42 36 50 30 53 51 63 58 50 60 54 33 48 40 76 61 46 65 65 15 45 58 40 64 18 33 40 64 18 33 40 48 9 38 41

Highest value observed: 76
Lowest Value: 9
Standard deviation from the ungrouped data: 15.25
Frequency:
Mean: 46.94
Standard deviation from grouped data: 15.40

standard deviation

calculate mean of a data set:

1. Turn calc. on or press MODE SeTUP
2.Press 3 for STAT
3. Press 1 for VAR
4.Enter values for X column
5. WHen done entering press SHIFT 1
6.Press 5
7.Press 2 to calculate the mean
8.Press =

Texas Instruments TI-86 Statistics How-To Information The following instructions should allow you to calculate the mean and standard deviation using a TI-86 calculator.

1. Press STAT
2. Select EDIT
3. Enter data into a list: [2.32 ENTER 2.43 ENTER 2.39 ENTER, etc.] Continue until all data have been entered. (use arrows keys to go to the y value, if any)
   
4. EXIT until you reach the main ("blank") screen. (If you try to calculate while you're in the data menu, it thinks you're still trying to enter data!)
5. Press STAT
6. Select CALC
7. Select OneVa
8. Select LIST, NAMES, xStat(F2), Press ENTER
9. The calculator provides Max, Min, Mean, Standard Deviation, Sum, Sigma, etc. You will be primarily interested in the Mean and Standard Deviation (sx).