2. Descriptive Statistics

Categorical variables

Measured in groups/ categories (nominal & ordinal)

Continuous variables

Measured on a continuous scale (interval & ratio)

Nominal

Values used to represent categories, but with no order

Ordinal

Values have an order/ hierarchy

Interval

Equal difference in value = equal difference in property, but 0 does not indicate abscence of property

Ratio

Equal difference in value = equal difference in property, but there is an absolute 0

Bar chart

Representation for nominal data (uses mode, frequencies, percentages)

Absolute frequency

Number of times a value is observed

Relative frequency (percentage)

Number of times a value is observed, relative to total number

Cumulative frequency

Add relative frequencies of a group to percentages of prev group

Mode

The value that occurs the most in categ. data

Histogram

Representation for interval/ratio data (uses mean, spread, standard deviation)

Spread

How much data values differ from each other and how much they differ from the measure of center

Standard deviation (sigma)

Measure of spread around the mean (square root of variance)

Standard deviation formula

Square root of (Sum of Squares devided by n)

Box plot

Representation for interval/ratio data (uses median, range, quartiles, IQR)

Median

The middle value when ordering values

Range

Maximum value - minimum value

Quartiles

Order values, find median (Q2), find medians of those halves (Q1, Q3)

Interquartile range (IQR)

Middle 50% of data, between Q1 and Q3

Scatterplot

Representation for interval/ratio data (useful for correlation coefficient)

Pearson’s r

Correlation coefficient, showing how closely points on scatterplot resemble a straight line