Can explain SPREAD to me on Slice the pie?

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I know you have to have ratings covering the whole spectrum of 1-10 every 30 songs. but i am a little confused, is that Every set of 30 songs or the past 30 you have listened to? I was doing quite well with mainly 3 stars but sometimes 2 but now have fallen to just one star and am finding it hard to raise my accuracy and stars! If anyone can help i would be very thankful