RMA Week 8: Descriptive Statistics II

the rest of the graphs section is too visual - use slides!

what are measures of spread and what are they measured by?

• how much the scores vary

• range

• interquartile range (IQR)

• standard deviation (SD)

how do you calculate the range

largest number - smallest number

what’s a disadvantage of the range

it tells you nothing about the scores between the minimum and the maximum

what does the IQR do

measures the spread of the middle 50% of scores, between the first and third quartiles.

what is the standard deviation

• the measure of variation around the mean.

• the typical deviation of the scores in your sample around the sample mean.

• gives an indication of what’s happening between the min and max of a data set

the higher the SD…

the larger the spread of the scores

step 1 of calculating the SD

calculate the sample variance.

1. find the mean

2. make a table for each value (x) minus the mean

3. square each value in the table above

4. sum the squared values

5. divide the summed, squared scores by the number of numbers -1

step 2 of calculating the SD

square root the variance

what can graphs be used for?

• visual methods of representing data

• indicates patterns within data

• can be used to decide to how to analyse data e.g. if outliers, use median, if bimodal, check for confounds like gender

• illustrate findings e.g. ppt presentation