Glossary
Normal distribution
Definition
A normal distribution is a symmetric, bell-shaped spread of values around a mean. Most values sit near the middle and fewer appear the farther you go out. IQ scores are scaled to follow a normal distribution, which is why they draw the familiar bell curve.
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Why it matters for IQ scores
Because IQ scores are standardized to a normal distribution with mean 100 and standard deviation 15 (Wikipedia: Intelligence quotient), every percentile and rarity figure on this site can be computed instead of surveyed. About 68% of people score between 85 and 115, about 95% between 70 and 130, and about 99.7% between 55 and 145 (the 68-95-99.7 rule).
An example
The tails thin out fast. From the curve, about 1 in 44 people score 130 or higher, about 1 in 740 score 145 or higher, and about 1 in 32,000 score 160 or higher. Each step of 15 points makes a score many times rarer than the step before.
Common confusions
The normal curve describes groups, not individuals, and it is least precise at the extremes where few people are measured. Very high and very low scores also carry more uncertainty, which is part of the ceiling effect problem. For the parts of the curve, see mean and standard deviation; for a full walk-through, read the IQ bell curve, explained, or browse the IQ score chart.
Try one · Abstract Reasoning
Which tile completes the grid?
Tap the option you think is right, or show the answer.
Show the answer
The missing tile has three outlined squares (option 5).
How to solve it
- Start with the first column and name what changes from tile to tile.
- Every row and every column contains each shape (star, square and triangle) exactly once.
- Every row and every column contains each number of shapes (three, one and two) exactly once.
- The fill (outline) stays the same in every tile.
- Check the rules on the second column, then apply them to the third.
- So the missing tile has three outlined squares.
That was 1 puzzle. The full test has 25.
Sources
- 68-95-99.7 rule. Wikipedia. en.wikipedia.org/wiki/68%E2%80%9395%E2%80%9399.7_rule
- Intelligence quotient. Wikipedia. en.wikipedia.org/wiki/Intelligence_quotient