<p>Diane, the score conversion for the "known" test was created in the same way, and does not necessarily have (and probably does not have) a normal distribution. The percentile ranks you've refered to are assigned after the converted score is determined; your earlier explanations assume the converted scores are assigned based on the percentile ranks. It is not true that 1% of scores are at each percentile; and, in fact, if this year's testers are really terrible compared to last year's, it's theoretically possible everyone who tests this year would get a rank in the bottom 50% (i.e., no one would score above 20). If testers get better and better each year, average ACT scores will rise.</p>
<p>There must be an "original" score conversion that was set when ACT first started doing score conversion and test form equating this way (which I think was in 1989), and it seems possible that a normal distribution was used to determine that. The only reason the conversion would look anything like a normal distribution now is if the abilities of testers hadn't changed over the years. But it's clear that the current percentile ranks do NOT reflect a normal distribution--it starts to get WAY off as you drop below a 28 or so.</p>
<p>The raw score is converted into a scale score using a table, and the percentile rank is then determined from the scale score using a different table. Both tables are created long before you test. So the percentile distribution does not affect the score you will receive. And, moreover, your score is unrelated to the scores of the others who tested at the same time or in the same year. If you bombed this April, it doesn't matter if everyone else bombed too. </p>
<p>The whole point of the way different test forms are equated is that a score of 30 today means the same thing as a score of 30 ten years ago--it reflects the same level of educational development--even if a 30 is at the 97th perecentile today versus the 90th percentile ten years ago. Its intent is actually the opposite of the sort of "grading on a curve" that makes your score meaningful only relative to your peers--which is what I believe testers are talking about when they ask "Is the ACT curved?"</p>