The normal curve is very specifically defined frequency polygon: a type of graph we introduced in chapter 3. This allows us to interpret the proportion in the table as probabilities. A probability in this context is simply the chance that any given case will have a certain value, or fall within a certain range of values. In other words, the probability that any given case from a normal distribution will have a value within 1 standard deviation from the mean is 0.6826. this way of interpreting the area under the normal curve as a probability will be especially will be especially useful in the following chapters on inference.