(i) IQ is an index of brightness.
(ii) It is the ratio of mental age to chronological age.
(iii) The concept of IQ was given by William. Stern who gave the formula to calculate IQ i.e.
• IQ is relatively stable.
• It is a good predictor of potential.
• IQ scores are distributed in a population in such a way that most people tend to fall in the middle range of the distribution.
•This can be shown in the form of following table.
**Classification of People on the Basis of**
| IQ Range | Descriptive Label | Percent in the Population |
| Above 130 | Very superior | 2.2 |
| 120-130 | Superior | 6.7 |
| 110-119 | High average | 16.1 |
| 90-109 | Average | 50.0 |
| 80-89 | Low average | 16.1 |
| 70-79 | Borderline | 6.7 |
| Below70 | mentally challenged/**** | 2.2 |
