Standard Deviation

The average (mean) tells you the center. But it hides the chaos.

Standard Deviation (SD) tells you the spread. It tells you how far the data points are from the average.

The Tale of Two Cities

City A has temperatures of 20°C, 20°C, and 20°C. Average: 20°C. SD: 0.

City B has temperatures of -20°C, 20°C, and 60°C. Average: 20°C. SD: Huge.

If you pack for City A based on the average, you are fine. If you pack for City B based on the average, you will die.

In Medicine

A drug might lower blood pressure by 10 points on average.

But if the SD is high, it means for some people it lowered it by 50 points (dangerous), and for others it raised it by 30 points (also dangerous).

A low SD means the drug is consistent. A high SD means it is a gamble.

The Diagnosis

Don't just look at the mean. Look at the spread.

Consistency is often more valuable than the average performance.