Mean is the arithmetic average of a distribution, obtained by adding the scores and then dividing it by the number of scores there are. Median is the middle score in a distribution; half the scores are above it and half the scores are below it. Mode is the most frequently occurring score in a distribution. A skewed distribution is when the distribution is leaning toward one side. It is the measure asymmetric probability and its real value of data. A skewed would be like a group of random people who take an IQ test all score above average would be skewed upwards. The three measures of central tendency, mean, median, and mode, would all be in the same place, the center of a distribution, on a normal distribution. In a positively skewed distribution the measures of central tendency will not be in the same place, they would be above the average, median, score.
In an intelligence test where the mean is 100 and the standard deviation is 15 are distributed in a certain way. Standard deviation is a computed measure of how the scores vary around the mean score. This means that 100 is the average score of all the scores and the scores vary about 15 points around the mean score.
If there are two groups, group one with a mean of 100 and group two with a mean of 115, there can be a higher score in group one than the mean of group two. A score in group one can be higher than 115 so long as it contributes to the average score of 100.
Norms for intelligence tests are periodically updated because more people have taken the test thereby updating the measures of central tendencies.
An intelligence test is bias when the questions are geared toward a particular subject and not all areas of intelligence. It can also be bias if the test is administered differently to different people.