Difference between highest & lowest values in data set. Describes data. Usually 1 added as correction
What is standard deviation?
Precise method of expressing dispersion. Measure of average distance between each data item above & below mean, ignoring plus or minus values. Shows variation in data- assesses spread of data around mean.
Evaluate range:
Easy to calculate but affected by extreme values. Fails to take account of distribution of numbers - doesn't indicate whether most numbers closely grouped around mean or spread out evenly.
Evaluate standard deviation:
Precise measure of dispersion as all exact values taken into account. Not difficult to calculate with calculator. May hide some characteristics of data set (e.g, extreme values).
To calculate standard deviation
1. Calculate mean of data set
2. Take mean away from each score in data set
3. Square each difference
4. Add together each of squared differences
5. Divide this by N-1 (number of scores in data set - 1)
6. Find square root of this value
Weaknesses of standard deviation:
Used if data set normally distributed, not skewed. Only used with data ordinal or above.
