Weighted Purchase Intent (also known as WPI) is calculation of the mean of a purchase intent scale when the values for each response category of the scale (called weights) reflect the probability of purchase for that response category. For a 5 point purchase scale the weights might be 80% for definitely will buy, 30% for probably will buy, 10% for might or might not buy, 3% for probability will not buy, and 1% for definitely will not buy. If the distribution on the scale was 40% definitely will buy, 30% probably will buy, 20% might or might not buy, 5% probably will not buy, and 5% definitely will not buy) then WPI = 80%*40% + 30%*30% + 10%*20% + 3%*5% = 1%*5% = 43.2%.
Different weights could be used (or different action standard norms could be used) depending on the field country or on the category or price of the item being evaluated. Different clients tend to have different weights and norms.