Which of these responses is NOT a standard score?

Standard scores are statistical measures used to express a value's position relative to the mean of a group of values. They are standardized to have a specific mean and standard deviation. The Z-score is one of the most common standard scores and indicates how many standard deviations an element is from the mean. Similarly, the T-score is another form of standardized score that changes based on the sample size but also represents how far a data point is from the mean in standard deviation units.

While standard deviation is a measure of variability that describes the spread of data in a set, it is not a standard score by itself. Therefore, it does not qualify as a standard score like the Z-score or T-score do.

Percentages, on the other hand, represent a fraction of a whole expressed in hundredths and do not relate to the concept of standardization in the same way standard scores do. They do not indicate how far a value is from the mean based on standard deviation units, hence they are classified as non-standard metrics. This distinction makes percentages the correct choice for being the only option that is not a standard score.