{"id":359,"date":"2009-07-22T21:26:02","date_gmt":"2009-07-22T20:26:02","guid":{"rendered":"http:\/\/moomango.co.uk\/ttfa\/?page_id=359"},"modified":"2009-07-22T21:26:02","modified_gmt":"2009-07-22T20:26:02","slug":"how-to-read-a-graph","status":"publish","type":"page","link":"https:\/\/i-base.info\/ttfa\/learning-resources\/how-to-read-a-graph\/","title":{"rendered":"How to read a graph"},"content":{"rendered":"

This short section explains how to read and understand information from a graph.<\/p>\n

What is a graph?<\/h2>\n

A graph is a way of showing complicated information in a clear easy to understand way.<\/strong><\/p>\n

They are used to summarise complicated results.<\/p>\n

What’s on a graph?<\/h2>\n

A graph usually has a vertical axis (y-axis) and horizontal axis (x-axis).<\/strong><\/p>\n

Each axis need to be clearly marked with what is being measured: time, CD4 count etc.<\/p>\n

An axis measures data<\/strong> (information or results) \u2013 and can measure anything.<\/p>\n

All graphs should have a clear title.<\/p>\n

\"Graph<\/p>\n

If time<\/strong> is one of the variables that is being compared, then this is always measured on the x-axis<\/strong>.<\/p>\n

If the graph is being used to show data<\/strong> rather than just a general trend or idea, then the units being measured<\/strong> need to be included on a scale<\/strong>, ie hours or years for time, cells\/mm3<\/sup> for CD4 counts.<\/p>\n

This scale needs to be shown in even measurements.<\/p>\n

If all the results cannot be fitted on the same scale, then the axis can be broken (as in the second graph above), and the scale shown on each section.<\/p>\n

Plotting data on a graph<\/h2>\n

Showing data on a graph is called plotting.<\/strong><\/p>\n

An example of how one person’s CD4 results after starting treatment could be plotted is shown in Figure 1<\/strong>.<\/p>\n

\"Figure<\/p>\n

To make results clearer, a line showing an average<\/strong> of the results is often added to make the general trend appear more clearly.<\/p>\n

Although the actual counts go up and down a lot, the average trend in the example above shows CD4 count increasing by about 200 copies\/mm3 <\/sup> over 18 months.<\/p>\n

You can also plot the average results of much larger amounts of data. For example the average CD4 counts of a group of 100 people after treatment could look exactly the same.<\/p>\n

The only difference in a graph that is showing more than one set of results, is that the numbers of people at each time point should also be included underneath each time. See Figure 2<\/strong>.<\/p>\n

\"Figure<\/p>\n

n = number<\/h2>\n

The mathematical term for number is n.<\/strong><\/p>\n

In Figure 2,<\/strong> the results are for a group of 100 people, but either not all the people have completed the study or some people have dropped out. You can see this by looking at the value for n underneath the x-axis.<\/p>\n

If the number of participants at the end of a study is much lower than at the beginning, you need to know what happened to the other people.<\/p>\n

Showing ranges and variations<\/h2>\n

Graphs should also show the range of variation within a group, not just the ‘average’.<\/strong><\/p>\n

This is shown by vertical lines that go up from and down from the average result that is plotted. See Figure 3.<\/strong><\/p>\n

\"figure
\n<\/strong><\/p>\n

The top and bottom of these lines often have a small horizontal line to make this clearer<\/p>\n

This can either show:<\/p>\n

    \n
  • The full range of the results<\/li>\n
  • The range of the middle half of the results (called the Inter-Quartile Range or IQR)<\/li>\n
  • The middle 95% of the results.<\/li>\n<\/ul>\n

    The graph should state which range is being shown.<\/p>\n

    Cautions about graphs<\/h2>\n

    Just as graphs can make information much clearer they can also be used to show make things look better or worse than they really are.<\/strong><\/p>\n

    Scales<\/strong>
    \nAlways check the scale on a graph. If it doesn’t start at zero, then the change shown may look more impressive than it really is.<\/p>\n

    Numbers of people or results at any time point<\/strong>
    \nIf a study started with 100 people, then any average results plotted in a graph should be an average of all 100 people. If early results of a study are being shown, the numbers at each time point may much lower after further time points.<\/p>\n","protected":false},"excerpt":{"rendered":"

    This short section explains how to read and understand information from a graph. What is a graph? A graph is a way of showing complicated information in a clear easy to understand way. They are used to summarise complicated results. …<\/p>\n","protected":false},"author":2,"featured_media":0,"parent":315,"menu_order":3,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-359","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/i-base.info\/ttfa\/wp-json\/wp\/v2\/pages\/359","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/i-base.info\/ttfa\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/i-base.info\/ttfa\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/i-base.info\/ttfa\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/i-base.info\/ttfa\/wp-json\/wp\/v2\/comments?post=359"}],"version-history":[{"count":0,"href":"https:\/\/i-base.info\/ttfa\/wp-json\/wp\/v2\/pages\/359\/revisions"}],"up":[{"embeddable":true,"href":"https:\/\/i-base.info\/ttfa\/wp-json\/wp\/v2\/pages\/315"}],"wp:attachment":[{"href":"https:\/\/i-base.info\/ttfa\/wp-json\/wp\/v2\/media?parent=359"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}