My experiment contains four block with similar trial sequence (but different backgrounds in each block thus making 4 different trial sequence). I have 240 word-pairs, one word pair appears in each trial. I have noticed that although I made a main block containing all the different blocks, the word pairs repeat themselves along the experiment. How can I make it so, that whatever words appear in the previous blocks will not appear again? meaning that when the experiment is finished 240 will have appeared once in all blocks.