For each descriptive word, we estimate P (w i |q img ), using topic proportions estimated using Equation 6: This is used to find I(w i ), a function of the likelihood of each word in the extracted caption: This function considers the prior probability of w i because frequent words often have a high posterior probability even when they are inaccurate.
the sum n i=1 δ i • I(w i ) is the overall measure of the correctness of a proposed caption conditioned on q img .