Solution

ID: past-exam-of-the-digital-social-research-statistics-core-course-of-the-university-of-oxford/2023/3/solution

Conditioning on marital status produces five contingency tables. In each, compare the conditional row percentages. The following differences are the sample proportion with some bad experiences minus the sample proportion without them, in percentage points; negative very-safe differences mean fewer respondents feeling very safe in the former group.
Marital-status stratumDifference: very safeDifference: unsafeReported Pearson's chi-squared test p-valueCramér's VGoodman and Kruskal's gamma
Single
Married
Living with a partner (inconsistent; see below)
Divorced
Widowed
The p-values test independence within the respective strata; the magnitudes and directions come from the conditional percentages and association measures. Differences in the table are calculated from the counts before rounding.
Among single respondents, the very-safe sample proportion changes from to , while the unsafe sample proportion changes from to . The direction resembles the weak pooled pattern, but supplies little evidence against independence in this stratum. This is not evidence that the population association is exactly zero.
Among married respondents, the very-safe sample proportion changes from to and the fairly-safe sample proportion rises from to . Unsafe remains almost unchanged, versus . Thus the within-stratum association again chiefly shifts very safe toward fairly safe. Its is below the significance level, while still indicates a small association.
For respondents living with a partner, very safe changes from to , fairly safe from to , and unsafe from to . The very-safe difference is larger than in the married stratum. The printed p-value would be just below the conventional significance level. However, this particular output is internally inconsistent: recalculating Pearson's chi-squared test from its six displayed counts gives , , and . The reported agrees with the counts, whereas the printed would itself imply for two statistical degrees of freedom, rather than the printed . Using the displayed counts, this stratum does not reject independence at . The source PDF itself contains these incompatible figures; this is not resolved by rounding.
Among divorced respondents, very safe changes from to , but unsafe also decreases, from to , as fairly safe increases from to . This is a redistribution toward the middle category, not a simple shift toward greater unsafety. The near-zero gamma, , reflects the lack of a consistent ordinal direction, while can still register distributional differences without a direction. The p-value does not reject independence.
Among widowed respondents, very safe changes from to , and unsafe from to . The ordinal association is positive on the chosen ordering, with gamma . Its lack of conventional statistical significance, , does not imply a smaller estimated association than in the married group: the widowed stratum has only complete observations, versus in the married stratum, and therefore less precision. All five tables have sufficiently large expected counts for the reported approximations; even the smallest expected count is about .
Figure 1.
Derived safety-category differences and unadjusted uncertainty by marital-status stratum
. Details.
The plot uses large-sample Wald confidence intervals for the difference of independent sample proportions within each stratum. They concern the individual category contrasts, whereas the tabulated Pearson's chi-squared tests compare the full three-category distributions. The intervals are unadjusted for multiple hypothesis testing; the pooled row reuses the stratum observations, so it is not a sixth independent sample.
In the elaboration model, the persistence of a generally weak positive ordinal pattern within four strata is closer to partial replication than to a complete explanation of the pooled association by marital status. The differences in estimated patterns suggest possible specification: the divorced group's shift toward the middle category differs from the widowed group's shift toward unsafe. They do not by themselves establish population effect modification. In particular, significant and nonsignificant results need not differ significantly; compare the effects using an interaction contrast or a fitted model with the appropriate interaction terms, rather than comparing their separate p-values.[ref]
One additional omnibus check can be derived from the supplied tables without assuming a common effect. Recalculating the five statistics from the counts, including the corrected living-with-partner statistic, the joint conditional independence test for stratified contingency tables gives
Under independent-stratum sampling this tests independence within every stratum jointly; it does not test whether the association is homogeneous across strata. It does not reject at , but this alone does not establish that stratification removed the population association: its ten-degree-of-freedom alternative is less focused than the pooled two-degree-of-freedom test, and nonrejection is not proof of equality.
The elaboration model also distinguishes an antecedent confounder from an intervening causal mediator. These cross-sectional tables do not establish the relevant time ordering or identify either role for marital status. Five separate stratum tests further require caution about multiple hypothesis testing: for example a Bonferroni correction at familywise would use per test, below all five individual p-values. That caution changes the interpretation of selected significance labels, not the observed conditional percentages.
The estimated patterns vary across strata, but the tables neither establish a population interaction nor prove that marital status explains away the association.

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