Measures of Spread
Measures of spread are ways of summarizing a group of data by describing how spread out the values are. If the spread of values in the data set is large, the mean is not as representative of the data as if the spread of data is small. This is because a large spread indicates that there are probably large differences between individual data points.
The spread of the values can be measured for quantitative data, as the variables are numeric and can be arranged into a logical order with a low end value and a high end value. Measures of spread are used in conjunction with a measure of central tendency, such as the mean or median, to provide an overall description of a set of data. To describe spread, a number of statistics are available, including the range, quartiles and standard deviation. Which you use will depend on how much and the type of data you collected.
What Measure of Central Tendency Did you Calculate? 

Mean with 5 or more trials per level of manipulation 
Standard Deviation 
Mean with 4 or less trials per level of manipulation 
Range 
Median with 5 or more trials per level of manipulation 
Quartile 
Median with 4 or less trials per level of manipulation 
Range 