Measures of dispersion

What is the range?

The difference between the highest and lowest score in the data set

What do measures of dispersion describe?

The spread of data, variation, around a central value

Strengths of the range? (2)

• Quick to calculate

• Confident way to express the spread of data

Weaknesses of the range? (2)

• Doesn’t consider the distribution of values around the centre

• Doesn’t take individual values into account (only the two most extreme scores), therefore making it very affected by extreme scores (outliers)

If a data set includes extreme scores, what is the best way to look at dispersion?

Through calculating the interquartile range, which involves cutting out the lowest quarter and highest quarter values and calculating the range of the remaining scores.

What is standard deviation?

Deviation refers to the distance of each value from the arithmetic mean. Each score in a data set would have its on deviation so we calculate a single value that represents all deviation scores - the standard deviation.

What does a large standard deviation tell us?

There was much variation around the mean

What does a small standard deviation tell us?

The data was closely clustered around the mean

What does zero standard deviation tell us?

All the data values were the same

Strength of standard deviation?

Precise way to measure the dispersion of the exact values being taken into account

Weakness of standard deviation?

Cannot be immediately sensed from the data, as opposed to the range, making it more difficult to calculate