MindMill Self Inventory (SI) personality measure draws upon the most up to date theoretical thinking on measuring personality. The MindMill Self-Inventory has been developed from the Big Five theory, the most modern and well validated personality theory. Professor Sidney Irvine, the creator of MindMill assessments worked closely with Professor Raymond Christal, who along with Ernest Tupes developed the original Big Five theory of personality. Prof Irvine continued to develop the Big Five based on recommendations - the inclusion of an extra domain of assertiveness, plus instant measures of social desirability and guarded response.