In this guide, I will show you how to log (log10) transform data in SPSS. I will also describe how to log transform data with a base other than 10.

# What is a log transformation?

Log transformation is used when data is highly skewed. Usually, log transformation is performed with a base of 10, hence the term ‘**log10**‘.

Understanding log transformation is best seen with an example. Let’s say we want to log10 transform the number ‘**100**‘. In other words, the base number (i.e 10 in this case) to the power of what number is needed to make 100? Since, 10 x 10 (i.e. 10^{2}) equals 100, then we can say log10(100) = 2.

Taking another example to elaborate on how log10 works, let’s work out what log10(1,000) is. Since 10 x 10 x 10 (i.e. 10^{3}) equals 1,000, then we can say log10(1,000) = 3.

# How to log (log10) transform data in SPSS

To log (log10) transform data in SPSS, simply follow the steps below. In this example, I have a variable containing 10 numbers called ‘**Data**‘.

1. In SPSS, go to ‘**Transform > Compute Variable …**‘.

2. In the ‘**Compute Variable**‘ window, enter the name of the new variable to be created in the ‘**Target Variable**‘ box, found in the upper-left corner of the window. I suggest calling this ‘**Log10X**‘, with X being the name of the original variable. In this example, my variable to be transformed is called ‘**Data**‘, so I am calling the newly transformed variable ‘**Log10Data**‘.

Then in the ‘**Numeric Expression**‘ window, type in ‘**LG10(X)**‘, with X being replaced with the name of the variable to be transformed. So, in this example, I will enter ‘**LG10(Data)**‘.

Then click the ‘**OK**‘ button to transform the data.

## The output

A new variable should now be present in your data sheet. This will be the newly log10 transformed data. Here is what my datasheet now looks like.

# How to log transform data with a different base in SPSS

Unfortunately, there isn’t a direct function to log transform data with a base other than 10 in SPSS. However, there is a way to do this by creating a formula. I will now show you how to do this.

1. Go to the ‘**Compute Variable**‘ window again by selecting ‘**Transform > Compute Variable …**‘.

2. Now it’s time to write the formula. Let’s say I want to log transform a variable with a base of 2 (instead of 10).

To do this, I will enter ‘**LN(Data)/LN(2)**‘ into the ‘**Numeric Expression**‘ window. Obviously, replace ‘**Data**‘ with the name of the variable to be transformed. Also, replace the ‘**2**‘ with the base of any other number that you want to log transform with.

Also, don’t forget to fill in the ‘**Target Variable**‘ box with the name of the new variable that will be created. In this example, I am calling the variable ‘**Log2Data**‘.

Finally, click the ‘**OK**‘ button to transform the data.

# Conclusion

In this article, I have explained step-by-step how to log transform data in SPSS. Usually, this is performed with the base 10, using the function ‘**LG10()**‘. However, other bases can be used in the log transformation by using the formula ‘**LN()/LN(base)**‘, where the base can be replaced with the desired number.

**SPSS version used:** 25