01 219.90 Kanjani∞ - Hesomagari / Koko ni shikanai keshiki
The NINKI ratings are the results of a POLL conducted by Oricon indicating degree of interest/degree of purchase intention in an upcoming release. Although the ratings are never a true reflection of sales, it does give a strong prediction about where the single/album will place in terms of popularity amongst the Japanese public. However, they are NOT an indication of how it will perform in terms of chart positioning and total sales for the week.
According to Oricon's note under the NINKI page, Oricon researches about 870,000 of its members by random sampling of 400 members in their 10s until 40s. 50 men and 50 women are chosen randomly to participate in each age group. (However, some popular artists have had NINKI exceeding 500, so we can only assume that Oricon has extended the polling to around 600/700 people now.)
Good luck to Eito!