The standard deviation is a measure of spread, in this case of IQ scores. A standard devation of 15 means 68% of the norm group has scored between 85 (100 – 15) and 115 (100 + 15). Also, 95% of the norm group has an IQ score within two standard deviations of the average.

How many standard deviations from the mean is normal?

The Empirical Rule states that 99.7% of data observed following a normal distribution lies within 3 standard deviations of the mean. Under this rule, 68% of the data falls within one standard deviation, 95% percent within two standard deviations, and 99.7% within three standard deviations from the mean.

What percentage is 2 standard deviations from the mean?

95%
Approximately 95% of the data fall within two standard deviations of the mean. Approximately 99.7% of the data fall within three standard deviations of the mean. Figure 8.8 below shows the percentage of normal data falling within one, two, and three standard deviations from the mean.

How do you interpret the standard deviation?

Low standard deviation means data are clustered around the mean, and high standard deviation indicates data are more spread out. A standard deviation close to zero indicates that data points are close to the mean, whereas a high or low standard deviation indicates data points are respectively above or below the mean.

Is high standard deviation good?

Standard deviation is a mathematical tool to help us assess how far the values are spread above and below the mean. A high standard deviation shows that the data is widely spread (less reliable) and a low standard deviation shows that the data are clustered closely around the mean (more reliable).

What is acceptable standard deviation?

For an approximate answer, please estimate your coefficient of variation (CV=standard deviation / mean). As a rule of thumb, a CV >= 1 indicates a relatively high variation, while a CV < 1 can be considered low. A “good” SD depends if you expect your distribution to be centered or spread out around the mean.

How does change in mean affect standard deviation?

When the smallest term increases by 1, it gets closer to the mean. Thus, the average distance from the mean gets smaller, so the standard deviation decreases. When the largest term increases by 1, it gets farther from the mean. Thus, the average distance from the mean gets bigger, so the standard deviation increases.

Is high standard deviation bad?

This means that distributions with a coefficient of variation higher than 1 are considered to be high variance whereas those with a CV lower than 1 are considered to be low-variance. Remember, standard deviations aren’t “good” or “bad”. They are indicators of how spread out your data is.