9. A formula is adopted which gives the probability of an individual of given ability passing a test item in terms of two quantities constant for that item. A method of estimating these two constants is given. Making certain assumptions concerning the items composing a test, formula? are then derived giving the expected value and the standard error of the test score of any person. It is shown that there is a certain reciprocity between persons and items, and corresponding formula? are given for the item scores. Finally, the results are applied to actual data obtained on a Moray House intelligence test, an estimate being made of the reliability of the test.
In conclusion I should like to thank Professor Godfrey H. Thomson for his help and valuable criticism in connection with this paper. I should also like to take this opportunity of thanking the Carnegie Trust for the Universities of Scotland for grants to cover the cost of the setting and printing of mathematical formula? in two papers previously published in the Society's Proceedings.