Banner tables
Significance Letters in Crosstabs, Explained
The letters on a market-research crosstab are column-proportions tests: every banner column gets a letter, and a cell shows the letters of the columns it is significantly higher than — uppercase at the stricter confidence level (usually 95%), lowercase at the looser one (usually 90%).
Published by crosstabs.com · Named statistical review pending · Last updated
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Suppose your banner has Gender (Male = A, Female = B) and Age (18–34 = C, 35–54 = D, 55+ = E). In the “Aware of brand” row, the Male column shows 42% B. That single letter says: among men, awareness (42%) is statistically significantly higher than among women, at the 95% confidence level.
If it showed 42% b (lowercase), the difference clears only the 90% bar — suggestive, not conclusive. No letter means no significant difference against any other column in that row.
How the test works
For each row, every pair of banner columns is compared with a two-proportion z-test on the column percentages. Because a table with five banner columns runs ten comparisons per row, a multiple-comparison correction is needed. Crosstabs uses Bonferroni: the significance threshold is divided by the number of eligible column pairs in that row. IBM SPSS Custom Tables documents Bonferroni and Benjamini–Hochberg options.
Columns with a test base under 30 are excluded, and ordinary tables are bounded to 12 banner columns and 250 row levels for letter generation. An untyped weight is rejected before letters are produced. Supported saved survey designs use design-aware association and uncertainty rather than ordinary weighted-count letters. See the survey-weighting guide before making inferential claims.
Run lettered tables free
Crosstabs computes lettered column comparisons free: open a CSV or XLSX in the workspace, build your crosstab, and switch on Letters in the display toggles. The letters survive CSV and Excel export, so your client deliverable keeps the notation.
How to interpret it
Rule of thumb
A letter answers one precise question: “is this column's percentage higher than that column's, beyond what sampling noise explains?” It says nothing about effect size — pair letters with the percentages themselves, and Cramér's V for the table overall.
Frequently asked questions
- What do the letters on a crosstab mean?
- Each banner column is assigned a letter (A, B, C…). Inside the table, a cell's letters list the columns whose percentage it is significantly higher than. For example, '34% B' under column A means column A's 34% is significantly higher than the same row's percentage in column B.
- Why are some letters uppercase and some lowercase?
- Most tools run the tests at two confidence levels. Uppercase letters mark differences significant at the stricter level (typically 95%), lowercase at the looser level (typically 90%). A lowercase letter is a weaker signal worth noting, not a confirmed difference.
- What statistical test is behind the letters?
- Crosstabs uses a pooled two-proportion z-test between every eligible pair of banner columns within each row, with a Bonferroni correction across those column pairs. IBM SPSS Custom Tables documents the same letter-key interpretation and offers multiple-comparison adjustments.
- Why do some columns show no letters at all?
- Crosstabs excludes test columns with a base below 30 and does not generate letters for ordinary tables above 12 banner columns or 250 row levels. No letter can also mean that no eligible difference cleared the corrected threshold; it does not prove equality.
References & further reading
- IBM SPSS Statistics 32 — Comparing column proportions
- IBM SPSS Statistics 31 — Custom Tables test statistics and multiple-comparison adjustments
- Agresti, A. (2013). Categorical Data Analysis, 3rd ed.
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