Graphs, Good and Bad

The distribution of a variable tells what values it takes and how often it takes these values Example Operations Performed at a Hospital last year Thoracic 20 Bones and joints 45 Eye, ear, nose, and throat 58 General 98 Abdominal115 Urologic 74 Neurosurgery 23 Other 65

Pie Chart – displays the division of a total quantity Used for qualitative data Should not include to many categories The number of degrees for each wedge should correspond to the percentage The total percentage must add to be 100%

Bar Graph – displays frequency or percentage of items in each category Used for one or more qualitative variables The bars can be vertical or horizontal The bars should be of uniform width and uniformly spaced The length of a bar represents the quantity we wish to compare

Example Cell Phone Subscribers (in millions of subscribers)

Line Graph (Time Plot) – shows the relationship between a quantitative variable and time Time is the horizontal scale (x-axis) The quantitative variable being measured is the vertical scale (y-axis)

Time series – a record of variable over time Trend – A steady change over time Seasonal component – the variable tends to be higher at certain points in time and lower at certain points in time All other variation can be explained by irregular cycles and random fluctuations

Increasing Trend

Decreasing Trend

Seasonal Variation

Increasing Trend with Seasonal Variation

Decreasing Trend with Seasonal Variation

No Trend and no Seasonal Variation