Surfstat.australia: an online text in introductory Statistics
SUMMARISING AND PRESENTING DATA
PRESENTING DATA FOR TWO CONTINUOUS MEASUREMENTS
So far we have considered only the presentation of results for a single
variable. In practice, most studies
are concerned with the association between two or more variables.
The appropriate statistical method depends on the situation.
- Two variables measured on the same
subject/object (e.g. peoples' heights and weights) - are these related?
correlation or regression;
- One variable measured at selected values of another variable -
(e.g. children's heights at different ages) Show simple trend - usually
- One variable measured at equally spaced times (or distances) but
successive values highly correlated or pattern complicated (e.g.
stockmarket prices) Use methods of time series analysis.
Scatterplots are useful for illustrating the relationship between
(xi, yi), i = 1,..n
A scatterplot is constructed by marking the scales of the two
variables, (x, y) along horizontal and vertical axes. Each pair of
measurements is plotted with a cross at the point using the
measurements as coordinates. The information from a third discrete
variable can be incorporated into the plot by using different plotting
symbols for each category of the third variable.
The figure below shows a scatterplot of income against age for some
hypothetical data. The number 2 is used to indicate a point where two data
Scatterplot of Income versus Age
Further information on sex is incorporated into this scatterplot
by using diamonds for males and squares for females.
Scatterplot of Income versus Age Classified by Sex
Example - Heights and weights of 6 female STAT101 students
The table below shows the heights and weights of 6 female STAT101 students.
How closely related are the heights and the weights?
|Student ||Height(cm) ||weight(kg) |
|1 ||167 ||60 |
|2 ||170 ||64 |
|3 ||160 ||57 |
|4 ||152 ||46 |
|5 ||157 ||55 |
|6 ||160 ||50 |
First step - draw a scatter plot:
The MINITAB command for a scatter plot is: