KNOWLEDGEBASE - ARTICLE #1487
Statistical Principles: The use and abuse of logarithmic axes
Logarithmic axes are widely used by students and scientists, and are a frequent source of confusion and frustration. To help make sense of log axes, we've created a 13 page article (table of contents below) explaining the uses and abuses of logarithmic axes. The article is written for users of GraphPad Prism, but almost all the information will be useful no matter how you make your graphs.
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What is a logarithmic axis?
- A logarithmic axis changes the scale of an axis.
- Interpolating between log ticks
- Why "logarithmic"?
- Lingo
- Other Bases
Logarithmic axes cannot contain zero or negative numbers
- The logarithms of negative numbers and zero are simply not defined
- A trick to plot zero on a logarithmic axis in Prism
- Logarithmic axes on bar graphs are misleading
When to use a logarithmic axis
- A logarithmic X axis is useful when the X values are logarithmically spaced
- A logarithmic axis is useful for plotting ratios
- A logarithmic axis linearizes compound interest and exponential growth
- An exponential decay curve is linear on a logarithmic axis, but only when it decays to zero
Lognormal distributions
- Plotting lognormal distributions on a logarithmic axis
- The mean and geometric mean
- Displaying variability on a lognormal distributions
Distinguish using a logarithmic axis from plotting logarithms
- Regression fits the data, not the graph
- Use antilog or powers-of-ten numbering when plotting values that are logs