Exchange blocks enable you to constrain permutations to occur within a group of observations, such as repeat measurments within the same individual.

There is an explanation of their use in the NBS manual. 

Originally posted by dylan sutterlin:

Hi ,

Thank you for this quick and helpful response/validation (and for a reminder that indeed not rejecting null hyp. is informative at some point)!

Can you explain what the exchange blocks do? I just realize that I used the example [1 2 3 4], but with only 3 participants in my example, so I am a bit confused about its usage. Let's say that my final design matrix has 60 rows (cond a x sub00-sub30 and cond. b x sub00-sub30; like the example above), and 31 columns (1/sub + the contrast one).

Then would the exchange block look like : [1 2 3 4 5 ...30 1 2 3 4 5 ... 30] ? 

Thank you very much,

Dylan