文档介绍：Individuals and Variables
Lecture 3 • Individuals are the objects described
by a set of data. Individuals may be
people, but they may also be animals
Relationship or things.
•A variable is any characteristic of an
Between Variables individual. A variable can take
different values for different
individuals.
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• The 1997 survey data set, for example,
includes data about a sample of women • The individuals described are the women.
at childbearing ages. Each row recodes data on one individual.
• The women are the individuals You will often see each row of data called
described by the data set. For each a case. Each column contains the values
individual, the data contain the values of of one variable for all the individuals.
variables such as date of birth, place of • Most data sets follow this format---each
residence, and educational level. row is an individual, and each column is a
• In practice, any set of data is variable.
panied by background
information that helps us understand
the data.
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1
Measuring center: the mean • To find the mean of a set of observations,
add their values and divide by the number
of observations. If the n observations are
• A description of a distribution almost
always includes a measure of its center or xx12, ,..., xn , their mean is:
average. The mon measure of
center is the ordinary arithmetic average,
xx12+ ++... xn
or mean. x =
n
Or in pact notation: 1
xx=
n ∑ i
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age at first marriage
Cumulative
Example: mean age at first Frequency Percent Valid Percent Percent
Valid 11 2 .0 .0 .0
12 2 .0 .0 .1
marriage 13 4 .1 .1 .2
14 13 .3 .3 .5
15 38 .8 .9
• Q105: When were you married for the first 16 111
17 185
time? 18 309
19 528
20 604
21 609
22 589
Statistics 23 435
24 307
25 191 9