I've just run a
statistical test on SPSS to see if there is a difference between articles in
the Guardian and Telegraph in terms of Characteristic X (it doesn't matter what
X is for my purposes here). The results are pasted below. The presence of X is
coded as 1, and its absence by 0.
The first table shows that a higher
proportion of Guardian articles (33.5%) than Telegraph articles (24.1%) had X.
The second table addresses the issue of statistical significance: can we be sure
that this is not a chance effect that would be unlikely to recur in another
sample of articles?
Paper * Code Crosstabulation
|
|||||
Code
|
Total
|
||||
.00
|
1.00
|
||||
Paper
|
Guardian
|
Count
|
121
|
61
|
182
|
% within Paper
|
66.5%
|
33.5%
|
100.0%
|
||
Telegraph
|
Count
|
60
|
19
|
79
|
|
% within Paper
|
75.9%
|
24.1%
|
100.0%
|
||
Total
|
Count
|
181
|
80
|
261
|
|
% within Paper
|
69.3%
|
30.7%
|
100.0%
|
||
Chi-Square Tests
|
|||||
Value
|
df
|
Asymp. Sig. (2-sided)
|
Exact Sig. (2-sided)
|
Exact Sig. (1-sided)
|
|
Pearson Chi-Square
|
2.322a
|
1
|
.128
|
.145
|
.083
|
Continuity Correctionb
|
1.898
|
1
|
.168
|
||
Likelihood Ratio
|
2.386
|
1
|
.122
|
.145
|
.083
|
Fisher's Exact Test
|
.145
|
.083
|
|||
N of Valid Cases
|
261
|
||||
a. 0 cells (0.0%) have expected count less
than 5. The minimum expected count is 24.21.
|
|||||
I decided I would like a two sided
significance level, and looked at the second table to find it. Unfortunately
there are no fewer than four different answers (0.128, 0.168, 0.122 and 0.145)! Which to choose?
Further study of the table only deepened my
confusion. The heading is Chi-Square tests but two of the columns are headed
Exact Sig. My understanding is that the chi-square test uses the chi-square
distribution which is a well known way of working out approximate
probabilities. The exact test works out the equivalent probabilities directly
without using the chi-square distribution, so the entries in the Exact test
columns are not chi-square results despite the table heading. One of the rows
is headed Fisher Exact Test and another Pearson Chi-Square which seems to
confirm this. But what can we make of the top right figure (0.083) which is
Chi-square according to the table heading, Pearson Chi-Square according to the
row heading, and Exact Sig according to the column heading? Help!
OK, I know I should have consulted the Help
(it doesn't work on my computer so I can't), or a book on using SPSS, or gone
on a course and provided employment for an expert. But I don't think this
should be necessary. SPSS should produce clear tables with a little explanation of what the numbers mean. In the present
case, as exact probabilities can be computed surely this is all that's needed.
With a sensible heading for the table, and a little note on what the
probabilities represent.
SPSS should produce clear, consistent
tables which present only the relevant information with an explanation in, as
far as possible, non-technical language.
But then people might understand the output
and the market for courses and experts would be much diminished.
