Menstrual cycle The first period usually begins between twelve and fifteen years of age, a point in time known as menarche. They may occasionally start as early as eight, and this onset may still be normal. The average age of the first period is generally later in the developing world and earlier in developed world. The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women and 21 to 35 days in adults (an average of 28 days). Menstruation stops occurring after menopause which usually occurs
Human action cycle Human action cycle The human action cycle is a psychological model which describes the steps humans take when they interact with computer systems. The model was proposed by Donald A. Norman, a scholar in the discipline of human–computer interaction. The model can be used to help evaluate the efficiency of a user interface (UI). Understanding the cycle requires an understanding of the user interface design principles of affordance, feedback, visibility and tolerance. The human action cycle describes how humans may form goals and then develop a series of steps required to achieve that goal, using the computer system. The user