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chi square
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2022-08-26
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THE CHI-SQUARE TEST
for 2X2 tables
The Chi-square test applies to
testing the independence of rows and
columns in a 2X2 table. Chi-square is
applied when certain conditions are
satisfied. They are as follows:
1) No cell has an EXPECTED value less
than five.
2) Never use percentages. Use actual
counts or frequencies.
3) The samples (classifications) are
independent. For example, if some
of the men and women in the
preceding article had been
married, independence would have
been violated, since a strong
relationship already exists
between husband and wife on
political party.
In the example of the previous
section, we ask whether the proportion
of males voting Democratic is the same
as the proportion of females voting
Democratic. Of course, we know that
60% (15/25) of the men in our sample
will vote Democratic while 41% (7/17)
of the females will vote Democratic.
So much for the obvious. A statist-
ical test of hypothesis helps to
answer the more general question:
IN THE ENTIRE COUNTRY, IS THE
PROPORTION OF MALES VOTING DEMOCRATIC
THE SAME AS THE PROPORTION OF FEMALES
VOTING DEMOCRATIC?
We know it is not true for our
sample, but is that sufficient? Maybe
we drew a weird sample.
Chi-square is a statistic which
helps to answer the question. It
tells us how likely it is that we
'just drew a weird sample'.
We deal numerically with 'likely' by
calculating the probability of an
event. The smaller the probability,
the less likely the event. Thus
if an event has a probability of .05
it is expected to happen 1 time in 20.
A probability of .01 means it is
expected to happen 1 time in 100, and
so on.
The probability associated with a
statistical test is referred to as the
'significance level of the test'.
----< continued in next article >-----