A relationship between two variables that can be described by a straight line. The normal or Gaussian distribution is one of the most important probability density functions, not the least because many measurement variables have distributions that at least approximate to a normal distribution. It is usually described as bell shaped, although its exact characteristics are determined by the mean and standard deveiation. It arises when the value of a variable is determined by a large number of independent prcoesses. For example, weigth is a function of many processes both genetic and environmental.
Many statistical tests make the assumption that the data come from a normal distribution.
Generally they are geometrical methods that attempt to present multivariate data in fewer dimensions. The difference between an observed and an expected or predicted value. The similarity between two objects is a measure of how closely they resemble each other. Dissimilarity is the inverse of this, and this is related to the concept of distance . Inevitably this can result data that have quite different ranges.