Yates' correction for continuity

Yates' correction for continuity, or Yates' chi-square test, adjusts the formula for Pearson's chi-square test by subtracting 0.5 from the difference between each observed value and its expected value in a 2 &times; 2 contingency table. This reduces the chi-square value obtained and thus increases its p-value. It prevents overestimation of statistical significance for small data. This formula is chiefly used when at least one cell of the table has an expected frequency less than 5.

Other sources say that this correction should be used when the expected frequency is less than 10.

Yet other sources say that Yates corrections should always be applied.