Graphical presentation of data and measures of central tendency

LECTU RE 2

Introduction

Summaries of data are presented in the results

section of reports, either in graphs, tables.

The presentation method used depends on the data

and the objectives of your study.

Graphs are usually labeled and figures and both

graphs and tables are numbered

Introduction

When deciding on how to present data, illustrate the most

interesting or important results (aspects of your report which you want your reader to focus on)

Avoid having too many pages of graphs or one table after

another

Both figures and tables should always have appropriate (brief)

title and be labeled so that they stand alone

When displaying your graphs and tables, always report your

summary statistics e.g. mean values, n, and one of the measures of dispersion; SD, Variance, or 95% confidence interval

Frequency Table

A table with one column listing events categories or

values of the variable (x) and the other column listing their respective frequencies (f) or the number of times the values occur Consider the following data for the levels of sulphur dioxide in the rainfall (ml/litre of rainwater): 0.7; 0.9; 0.9; 0.7; 0.8; 0.7; 1.0; 0.8; 0.8; 0.9; 0.7; 1.2; 0.6; 0.5; 0.8; 1.0; 1.0; 0.8; 0.6; 0.8 ∑f =n = 20

Frequency table

x 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 f 1 2 4 6 3 3 0 1

Frequency table

Where the data may be a large number of values,

display data in classes of equal sizes e.g. 0.40-0.49; 0.50-0.59; 0.60-0.69 etc. This is called group frequency distribution

Number of classes (Generally recommended to use

5-20 classes)

Width of classes: Related to number of classes

Approx. class width = [Largest Data value- Smallest ]/Number of classes

Frequency table

May reveal the following:

The range of frequencies observed Approximately where most points lie

Variability among frequencies- do all values listed in...