One of the first things that you may wish to do when you have entered your data into a computer is to summarize them in some way so that you can get a ‘feel’ for the data. This can be done by producing diagrams, tables or summary statistics (Chapters 5 and 6). Diagrams are often powerful tools for conveying information about the data, for providing simple summary pictures, and for spotting outliers and trends before any formal analyses are performed.
One Variable
Frequency Distributions
An empirical frequency distribution of a variable relates each possible observation, class of observations (i.e. range of values) or category, as appropriate, to its observed frequency of occurrence. If we replace each frequency by a relative frequency (the percentage of the total frequency), we can compare frequency distributions in two or more groups of individuals.
Displaying Frequency Distributions
Once the frequencies (or relative frequencies) have been obtained for categorical or some discrete numerical data, these can be displayed visually.
- Bar or column chart – a separate horizontal or vertical bar is drawn for each category, its length being proportional to the frequency in that category. The bars are separated by small gaps to indicate that the data are categorical or discrete (Fig. 4.1a).
- Pie chart – a circular ‘pie’ is split into sectors, one for each category, so that the area of each sector is proportional to the frequency in that category (Fig. 4.1b).
Figure 4.1 A selection of diagrammatic output which may be produced when summarizing the obstetric data in women with bleeding disorders (Chapter 2). (a) Bar chart showing the percentage of women in the study who required pain relief from any of the listed interventions during labour. (b) Pie chart showing the percentage of women in the study with each bleeding disorder. (c) Segmented column chart showing the frequency with which women with different bleeding disorders experience bleeding gums. (d) Histogram showing the weight of the baby at birth. (e) Dot plot showing the mother’s age at the time of the baby’s birth, with the median age marked as a horizontal line. (f) Scatter diagram showing the relationship between the mother’s age at delivery (on the horizontal or x-axis) and the weight of the baby (on the vertical or y-axis).
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
