Final answer:
In a chi-square test of independence, no more than 20% of the cells should have expected frequencies less than 5 to ensure the validity of the test.
Step-by-step explanation:
When using a chi-square test of independence, it is important that each expected frequency in the cells of the contingency table be at least five to ensure the validity of the test. This requirement is because the chi-square test is based on the assumption that the distribution of the expected frequencies is close to the true distribution. When more than 20% of the cells have expected frequencies less than five, or any cell has an expected value less than one, the test may not be appropriate, and a different statistical test or a data transformation may be necessary.