10/10/08 19:54:27
>>413 すまぬ。Item Discriminationのところは完全に間違ってた。
Item Difficulty
The item difficulty for the TOEIC Listening and Reading test is characterized by a statistical
index called delta (Δ). Delta is defined as 13 + 4z, where z is the normal deviate corresponding
to proportion correct. Delta values ordinarily range from 6.0 for a very easy item
(i.e., approximately 95% of test takers select the correct answer) to 20 for a very hard item
(i.e., approximately 5% of test takers select the correct answer) with a mean of 13.0 (50% correct).
Item Discrimination
The item discrimination for the Classic and Redesigned TOEIC Listening and Reading test is
indicated by a statistical index known as the R-biserial correlation coefficient.
Like the item difficulty index delta,the R-biserial is an item-level statistic that indicates
the correlation between test-takers’ scores on a particular item (e.g., 0s or 1s on an item)
and the corresponding total scores (e.g., total scores for a TOEIC Compendium 4.6 section).
The R-biserial correlation shows how well an item differentiates between high and low ability
test takers.