A normal curve is a theoretical distribution. The smaller the sample size, the greater we can expect deviation from percentages associated with the normal curve. Even so, the theory holds true (statistically) when there are minor violations to the shape assumption. This means it is useful for interpreting my data as long as it’s approximately normally distributed.
When a distribution is normal, 99% of the sample points will fall within 3 standard deviations of the sample mean. Standard Deviation is the average linear distance from the mean = square root of the variance.
|normal distribution curve|