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How to Read Log Scale Graphs

by Stephanie Ellen, Demand Media

    A regular graph has numbers spaced at even intervals, while a log scale graph has numbers spaced at uneven intervals. The reason for this is that while a regular graph uses regular counting numbers like 1,2,3,4, and 5, a logarithmic graph uses powers of 10, such as 10, 100, 1000 and 10,000. To add to the confusion, scientific notation is often used on log scale graphs, so instead of 100 you might see 10^2. Reading a log scale graph is no more challenging than reading a regular X Y axis graph.

    Step 1

    Locate the point on the X axis that you want to take a reading for.

    Step 2

    Find the corresponding point on the Y axis. Draw an imaginary vertical line with your finger up to the graph and then draw an imaginary line to the left until you cross the vertical axis. This is your Y axis reading.

    Step 3

    Convert the number from scientific notation if necessary. For example, if the reading is 10^2, the actual number is 1,000.

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    Tip

    • Although the Y axis is usually the logarithmic scale, the Y axis and X axis may be transposed on some graphs. In other words, the logarithmic scale may be on the X axis and not the Y axis. You can tell which is which by looking for powers of 10 on the axis.

    Warning

    • When reading logarithmic graphs, remember that you are using a logarithmic scale. One common error that students make when reading logarithmic graphs is to see a line graph and assume that there is a linear relationship. While a line in a regular numbered graph means a linear relationship, in a logarithmic graph it normally means an exponential relationship.

    About the Author

    Stephanie Ellen teaches mathematics and statistics at the university and college level. She coauthored a statistics textbook published by Houghton-Mifflin. She has been writing professionally since 2008. Ellen holds a Bachelor of Science in health science from State University New York, a master's degree in math education from Jacksonville University and a Master of Arts in creative writing from National University.

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