# The mean

The mean

The mean - formula
What is the Mean? The mean or average gives an average of a series of numbers. For example, the average body weight of my friends. The formula for calculating the mean is

$$\bar{X} ={\sum{X} \over {n}}$$

X with the line on top we read: X bar (eks bar). It sands for the mean, the average of the scores in our sample.
X stands for score.
The capital Greek letter we read: Sigma. It stands for the Sum of the scores.
The n we read: n. It stands for the number of scores in our sample.

The mean -practice example
An experimenter wanted to know what quantity of food young rats eat. weighed the food each ate in 24 hours in grams. He chose 6 rats randomly from the colony of rats in his lab. Here are the data, The symbol for each score is X.

X
34
35
39
30
39
40

Looking at the formula above, we add all of scores and divide by the number of scores. The symbol for the number of scores is n. In this case the n is 6. The sum of the scores is 217, ΣΧ=34+35+39*30+39+40=217 Now we divide this by 6, the n, and we find the mean 36.17.

$$\bar{X}= {217 \over{6}}= 36.17$$

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