Contingency tables — Edexcel A-Level Statistics
Test yourself on Contingency tables with PEARSON EDEXCEL A-Level practice questions.
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Contingency tables explained
A contingency table cross-tabulates two categorical variables, with rows for one variable's categories and columns for the other's, and cell counts showing how many observations fall into each combination.
Read the full explanation
To construct one from real data, decide the two variables, list their categories, and tally each observation into the correct cell; add row and column totals to give marginal frequencies. Combining categories is appropriate when some cells have very small counts, when categories are too fine to interpret, or when a category is not relevant to the question, but the combination must be logically meaningful and applied consistently to both variables.
Your focus
- Construct a contingency table from raw categorical data, including marginal totals.
- Combine categories appropriately and justify the combination.
- Interpret patterns in a contingency table in the context of the data.
Contingency tables exam tips
Marking Points
- A contingency table displays counts for two categorical variables, with one variable defining rows and the other defining columns.
- Each observation is tallied into exactly one cell, and row and column totals are included as marginal frequencies.
- Categories may be combined when cell counts are too small or categories are too detailed, provided the combination is meaningful and consistent.
- Interpretation compares observed counts or percentages across rows or columns in the context of the variables studied.
- Percentages should be calculated within an appropriate total (row or column) depending on the comparison being made.
- An apparent association in a contingency table does not by itself establish a causal relationship.
Examiner Tips
- 💡Label both variables and their categories clearly, and include row and column totals in the table.
- 💡When combining categories, state the reason and check that the combined categories remain meaningful.
- 💡In interpretation questions, quote relevant counts or percentages and relate them to the real-world context given.
Common Mistakes
- Placing an observation in more than one cell or omitting it entirely. Correction: each observation belongs to exactly one row-and-column combination.
- Combining categories inconsistently, such as merging two rows but not the corresponding columns. Correction: apply any combination rule consistently across the table.
- Calculating percentages using the grand total when a row or column comparison is intended. Correction: choose the denominator that matches the comparison being made.
- Treating a strong association in the table as proof of cause and effect. Correction: describe the association and note that causation requires further evidence.