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Applied Statistics Using SPSS, STATISTICA, MATLAB and R

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5.2 Contingency Tables 197<br />

first categorise the SCORE variable into four categories. These can be classified<br />

as: “Poor” corresponding to a final examination score below 10; “Fair”<br />

corresponding to a score between 10 <strong>and</strong> 13; “Good” corresponding to a score<br />

between 14 <strong>and</strong> 16; “Very Good” corresponding to a score above 16. Let us call<br />

PERF (performance) this new categorised variable.<br />

The 3×4 contingency table, using variables PROG <strong>and</strong> PERF, is shown in Table<br />

5.13. Only two (16.7%) cells have expected counts below 5; therefore, the<br />

recommended conditions, mentioned in the previous section, for using the<br />

asymptotic distribution of T, are met.<br />

The value of T is 43.044. The asymptotic chi-square distribution of T has<br />

(3 – 1)(4 – 1) = 6 degrees of freedom. At a 5% level, the critical region is above<br />

12.59 <strong>and</strong> therefore the null hypothesis is rejected at that level. As a matter of fact,<br />

the observed significance of T is p ≈ 0.<br />

Table 5.13. The 3×4 contingency table obtained with <strong>SPSS</strong> for the independence<br />

test of Example 5.12.<br />

PERF Total<br />

Poor Fair Good Very<br />

Good<br />

PROG 0 Count 76 78 16 7 177<br />

Expected Count 63.4 73.8 21.6 18.3 177.0<br />

1 Count 19 29 10 13 71<br />

Expected Count 25.4 29.6 8.6 7.3 71.0<br />

2 Count 2 6 7 8 23<br />

Expected Count 8.2 9.6 2.8 2.4 23.0<br />

Total Count 97 113 33 28 271<br />

Expected Count 97.0 113.0 33.0 28.0 271.0<br />

The chi-square test of independence can also be applied to assess whether two<br />

or more groups of data are independent or can be considered as sampled from the<br />

same population. For instance, the results obtained for Example 5.7 can also be<br />

interpreted as supporting, at a 5% level, that the male <strong>and</strong> female groups are not<br />

independent for variable Q7; they can be considered samples from the same<br />

population.<br />

5.2.4 Measures of Association Revisited<br />

When analysing contingency tables, it is also convenient to assess the degree of<br />

association between the variables, using the ordinal <strong>and</strong> nominal association<br />

measures described in sections 2.3.5 <strong>and</strong> 2.3.6, respectively. As in 4.4.1, the

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