Mean, Median, Mode

Mean: Sum of all values divided by count. Affected by outliers.

Median: Middle value when sorted. NOT affected by outliers.

Mode: Most frequent value. Rarely tested.

Range: Highest minus Lowest

Standard deviation: Measures spread. Larger spread = larger SD. Tighter cluster = smaller SD. You won't calculate it.

SAT question: "Which measure would change more if an outlier were removed?"

Answer: The mean. Median is resistant to outliers.

Weighted average trap:

Class A: 20 students, avg 80. Class B: 30 students, avg 90.

Combined is NOT 85. It's (20x80 + 30x90) / 50 = 86.

Reference:

Wikipedia: Mean

https://en.wikipedia.org/wiki/Mean