Descriptive statistics

measures of central tendency

mean, median, mode

mean

‘average’

calculated by adding up all the scores in the data set then dividing by the number of scores

strength of mean

most representative of all the measures of central tendency because it is comprised of the whole data set

limitation of mean

most sensitive measure as outliers (extreme scores) can distort the mean - only used with ordinal and interval data

median

‘middle’

calculated by putting all scores in rank order from smallest to largest then selecting the middle number from the data set

strength of the median

not distorted by extreme scores

limitation of the median

does not reflect all scores in the data set - less sensitive

mode

‘most often’

calculated by identifying the most frequently occurring score within the data set

strength of the mode

not distorted by extreme scores - can be used with nominal data unlike the other methods

limitation of the mode

there can be more than one mode so it is not always useful

measures of dispersion

range and standard deviation (SD)

range

calculated by subtracting the lowest score in the data set from the highest score in the data set

standard deviation

looks at how far the scores deviate from the mean

The larger the SD, the more spread out the data is

strength of the range

easy to calculate

limitation of the range

does not indicate the distribution pattern across the whole data set

strength of standard deviation

is a precise measurement of dispersion because all values in the data set are included in the calculation

limitation of standard deviation

extreme values can distort the measurement

percentage formula

change / original x 100