normal distribution

n. A class of probability distributions that are depicted graphically as a unimodal (singlepeak), symmetrical, bell-shaped density curve with the majority of scores in the middle of the distribution and fewer scores near the ends of the distribution (the ends of the curve extend to infinity). While the area under a normal distribution curve is always constant (equal to 1), the height and spread of the curve vary as a mathematical function of the mean and standard deviation. If data is drawn from a normally distributed population, 68% of the values are within one standard deviation of the mean, 95% are within two standard deviations, and 99.7% are within three standard deviations (this is known as the 68-95-99.7 rule or the empirical rule). As such, the vast majority of scores on a normal distribution will fall within three standard deviations from the mean. While there are many possible configurations for a normal distribution based on various means and standard deviations, a standard normal distribution always has a mean of 0 and a standard deviation of 1. – BJM