Mitchell's thinking was that in rssttt there was the nice alphabetical progression, so people might go "obviously rssuuu then" whereas with mrrjjj there isn't really a pattern with the letters, so people will have to search harder for the idea, and might eventually settle on mrrjjjj. However, we only got one mrrjjjj - and an mrrnnnn - so that thought didn't really carry over to my convenience sample.
I found abc -> abd, mrrjjj -> ? the most interesting case, and was determined to find a sensible pattern in it. My answer of mrrnnnn was motivated by the pattern I decided upon: here, abc is paralleled by mrrjjj in the sense that each is composed of three members of a series: (a)(b)(c) and (m)(rr)(jjj). In abc one adjusts the initial member a to form each later member by [1] incrementing the alphabetic position by +1, +2, +3 etc. and [2] incrementing the number of letters composing the member by 0. In mrrjjj one adjusts the initial member m to form each later member by [1] incrementing the alphabetic position by +5, -3, +1 etc. and [2] incrementing the number of letters composing the member by +1, +2, +3 etc. The transformation abc -> abd means advancing the third member by one place in the series, and so I applied the same transformation to mrrjjj: the fourth member would increment m by +1 alphabetically and increment the number of identical letters composing it by +3, therefore mrrnnnn. (I followed a similar rationale for the simpler abc -> abd, rssttt -> ?, and in that case my answer was shared by two others.)
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I found abc -> abd, mrrjjj -> ? the most interesting case, and was determined to find a sensible pattern in it. My answer of mrrnnnn was motivated by the pattern I decided upon: here, abc is paralleled by mrrjjj in the sense that each is composed of three members of a series: (a)(b)(c) and (m)(rr)(jjj). In abc one adjusts the initial member a to form each later member by [1] incrementing the alphabetic position by +1, +2, +3 etc. and [2] incrementing the number of letters composing the member by 0. In mrrjjj one adjusts the initial member m to form each later member by [1] incrementing the alphabetic position by +5, -3, +1 etc. and [2] incrementing the number of letters composing the member by +1, +2, +3 etc. The transformation abc -> abd means advancing the third member by one place in the series, and so I applied the same transformation to mrrjjj: the fourth member would increment m by +1 alphabetically and increment the number of identical letters composing it by +3, therefore mrrnnnn. (I followed a similar rationale for the simpler abc -> abd, rssttt -> ?, and in that case my answer was shared by two others.)