Download Lecture 5 - Vanderbilt

Lecture 5
The center of symmetric distributions: the mean
•Besides the median, there is one more
good measurement of the “center”
•It works especially well if the distribution is
symmetric
𝑇𝑜𝑡𝑎𝑙
𝑛
𝑛
𝑖=1 𝑦𝑖
•𝑦 =
=
𝑛
collected values
- the average of all
Example
75.60946 47.67863 13.4834 68.27895
67.70208 27.63657 10.97026 52.80463
86.75896 35.4131 92.91208 89.27507
50.14048 13.405 49.27678 65.42801
13.67298
Median ≈50.14
The mean is 50.61
Interpretation of mean (or average)
•“Center of mass”: if we have points with
assigned masses, the “center of mass” is
•In the same way, the mean is a point
where the histogram balances
Mean or median?
•This is a tough question. For many
“scientific” purposes, the mean is a lot
better than the median. The notion of
“Expectation” (which is a generalized
mean) is central in Probability and
Statistics
Mean or median vol. 2
•However, in some situations median is
more “stable”. For example, if we collect
ages of students in a class, and typically
it’s, say, 18, 19, 20; but then there is a
70 y.o. student.
18,18,18,18,18,19,19,19,19,19,20,20,20,
20,20, 70
With or without the 70, median is = 19.
With the 70, mean is = 22.19
Without the 70, mean is = 19
So with the obvious outlier, mean does
not represent an “average” student.
•The reason behind this situation is not
that the distribution is not symmetric. In
fact, it is not symmetric in a special way
Draw a picture!
•It always helps to draw a good picture
and look at the histogram. It often
clarifies, should we trust mean or
median (or both).
•Sometimes people throw away top and
bottom 10% of the data and average the
rest.
The spread: the standard deviation
•This should tell us how far actual values
are from the mean, in average
(𝑦𝑖 −𝑦)
.
𝑛
•Take
This is always equal to 0
•The reason is: some of the terms are
positive, and some are negative. Since the
mean perfectly balances things, they add
up to 0
Staying positive
•To destroy all negative terms, we square
them.
2
•𝑠 =
2
•𝑠
𝑦𝑖 − 𝑦 2
𝑛
2
or 𝑠 =
𝑦𝑖 −𝑦 2
𝑛−1
is called the “variance”, and 𝑠 = 𝑠 2 is
called “standard deviation”
•It is certainly correct to divide by n, but the
book suggests to divide by n-1.
Example
•14, 13, 20, 22, 18, 19, 13
•First find the mean:
(14+13+20+22+18+19+13)/7 = 17
•Now find (data value – mean):
14-17=-3, 13-17=-4, 20-17=3, 22-17=5,
18-17=1, 19-17=2, 13-17=-4
•14, 13, 20, 22, 18, 19, 13, mean = 17
14-17=-3, 13-17=-4, 20-17=3, 22-17=5,
18-17=1, 19-17=2, 13-17=-4
2
2
2
2
2
2
• −3 + −4 + 3 + 5 + 1 + 2 +
2
−4 = 9 + 16 + 9 + 25 + 1 + 4 + 16 =
80
•Now divide by 7 (book suggests 7-1=6)
•80/7 ≈11.43, 80/6 ≈13.33 – this is the
variance. St. dev. = square root ≈3.38 (3.65)
Glance into future: why do we need all that?
•We will rely on the following fact: if the
distribution is “normal”, then most of the data
should be between
(mean – 3*st.dev.) and (mean+3*st.dev.)
That is, if we know that our distribution is nice,
then it is more or less enough to know only
mean and standard deviation
•On professors side, most of the scores
(in a single test and total) should fall into
mentioned interval
•In a sense, “curve” means that professor
adjusts the scores to make this happen.
Why it’s good to pay attention
•On the quiz tomorrow you will be
allowed to use a calculator. Please bring
one!
Understanding and comparing distributions
Magnitudes only
•South America: 6.7 8.2 7.6 5.1 4.9 7.1
8.3 5.3 6.9 7.6 7.6
Median = 7.1 Mean = 6.84
•North America: 5.1 7.2 6.4 7.9 6.9 6.1
6.3 6.0 6.9
Median = 6.4
Mean = 6.53
The distribution for North America is more
symmetric; also in South America a typical
magnitude is higher