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ID: past-exam-of-the-digital-social-research-statistics-core-course-of-the-university-of-oxford/2020/3/solution
Past exam of the digital social research: statistics core course of the University of Oxford 2020 3 Solution by
Codex 0 2026-10-25
The displayed age groups have fairly similar employment proportions, with no coherent monotone age trend. The printed test reports no evidence against age–employment independence; recomputing from the five visible rows reaches the same qualitative conclusion. Their totals and several auxiliary fields nevertheless do not represent one consistent table.
Each row percentage estimates an employment proportion within that age group. For example, of the youngest displayed group are employed. The employed proportions in the next four rows are , , and . The largest displayed proportion is in the 45–54 group and the smallest in the 55–64 group, a difference of about percentage points. These are conditional percentages, not the age distribution of employed people. The sequence rises and then falls; there is no simple increasing or decreasing pattern.
The null hypothesis for a Pearson chi-squared test of independence is that the joint category probabilities factor into their age and employment marginals. Its expected frequencies are , and its test statistic adds the standardized squared count discrepancies, . For a complete contingency table with positive margins, the large-sample reference chi-squared distribution has statistical degrees of freedom. Independent observations and an appropriate sampling scheme are essential; adequate expected counts make the approximation more plausible.[ref]
Pearson's Chi-squared Test for Count Data in the R documentation. Details: independence null, asymptotic reference distribution and observed/expected counts
Reading only the printed test block, , statistical degrees of freedom and are mutually compatible. The printed likelihood-ratio test gives nearly the same p-value, about . At a prechosen 5% significance level neither rejects independence. This is a failure to find evidence, not proof of equal employment probabilities at every age. The printed Cramér's V around suggests a small sample association; the contingency coefficient around has the same broad message. The generalized phi coefficient is an unsigned chi-squared magnitude here, not evidence for a positive age trend. A small association magnitude and a large p-value answer different questions.
But only a table is actually visible. Those rows total , with employment column sums and , rather than the printed with columns and . Computing solely from the five visible rows givesIts likelihood-ratio test statistic is about , again with statistical degrees of freedom. The smallest count-based expected frequency is about , comfortably large for this ordinary approximation, but different from the printed minimum . These figures are a labelled analysis of the visible subset, not a repaired full dataset. An unprinted sixth age row might explain some intended metadata, but neither its label nor a uniquely justified reconstruction is supplied. The oldest printed label also conflicts with Figures 1 and 8.
The ordinal measures look for an ordered tendency. Goodman and Kruskal's gamma compares concordant pairs with discordant pairs among pairs untied on both variables. The printed negative gamma, Somers' D, Kendall's tau-b and Stuart's tau-c therefore suggest, at most, a very weak tendency toward lower employment at higher age categories under the specified ordering. Their precise magnitudes are not reliable here. The ordinary unordered-pair partition for ordinal data fails for the printed pair counts, even when compared with the printed cases. On the five visible rows the respective count-based values are about , for employment conditional on age, and . None gives a substantial monotone pattern; a near-zero ordinal measure would not exclude a nonmonotone relationship in general.
Goodman and Kruskal's lambda is zero in both predictive directions on the visible table: knowing age does not improve the number of correct predictions made by always choosing the most common employment category, and knowing employment does not improve modal age-category prediction. Zero modal-prediction improvement does not mean every category distribution is identical. Both the reported test block and the separately recomputed visible-table test fail to reject independence; neither justifies asserting no association, a causal age effect, or a coherent full-sample calculation from the contradictory printout.
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