In this study, we describe what factors influence the observed score correlation between an (external) anchor test and a total test. We show that the anchor to full‐test observed score correlation is based on two components: the true score correlation between the anchor and total test, and the reliability of the anchor test. Findings using an analytical approach suggest that making an anchor test a miditest does not generally maximize the anchor to total test correlation. Results are discussed in the context of what conditions maximize the correlations between the anchor and total test.