2. Variability

Question 1

Define ‘variance’

Answer 1

How spread out the scores are around the mean

Question 2

What is standard deviation?

Answer 2

The average amount that scores differ from the mean

Question 3

What are the two ways to express variability?

Answer 3

Visual depiction
- e.g. histograms, amount and shape of spread

Numerical info about variable

Question 4

Identify the two ways of assessing variability

Answer 4

Range

(Standard) Deviation

Question 4

Steps to calculate standard deviation

Answer 4

1) list scores in vertical line

2) add up scores

3) calculate mean

4) calculate each score’s deviation from mean

5) square each deviation

6) add up all squared values (sum of squares, aka SS)

7) variance score = SS/ number of scores

8) sd = square root of variance

Question 4

Deviation

• what is it?
• how do we calculate it?

Answer 4

the distance from the mean
(X - M)

mean deviation always zero
so we square the value

Question 4

Range

Answer 4

Interquartile range
Semi-inter-quartile range

Both affected by extreme scores

Question 5

Name the types of distributions

Describe what their curves would look like

Answer 5

Negative - tail to left (left foot)

Normal

Positive - tail to right (right foot)

Bimodal - multiple peaks

Question 5

What is the difference between variation and standard deviation

Answer 5

variance (SD^2) - mean squared deviation

sd - square root of variance

Question 6

Calculation for “sum of squares”…

Answer 6

ss = Σ(X - M)^2

sum (value - mean)squared

Question 7

Calculation for variance…

Answer 7

SD^2 = Σ(X - M)^2 /N

sum (value - mean)squared
divided by total num of values

Question 8

Calculation for standard deviation…

Answer 8

SD = square root of variance

SD = √ (Σ(X - M)^2 /N)