Product: TIBCO Spotfire®

**Problem:**

Regression Modeling with a logarithmic term

**Solution:**

If a linear regression model (generated using "Tools > Linear Regression..") contains a "log(x)" term, which of the functions in the Spotfire expression language should be used when building a calculated column that uses this logarithmic term?
When a linear regression model contains a "log(x)" term, the Spotfire expression language's "Ln()" function should be used in generating a calculated column that uses this logarithmic term.

Explanation:

In the background, the "Tools > Linear Regression..." feature in the Spotfire desktop client's main menu is implemented using a Spotfire data function that calls the Spotfire desktop client's built-in TIBCO Enterprise Runtime for R (TERR) statistical engine. In TERR, the log() function is the natural logarithm, the log10() function is the base-10 logarithm, and the logb() function is the user-defined-base logarithm, as shown in the following examples.

>

> log( 10 )

[1] 2.302585

>

>

> log10( 10 )

[1] 1

>

>

> logb( 10, 2 )

[1] 3.321928

>

In the expression language used for calculated columns in Spotfire itself, the corresponding functions are Ln() for the natural logarithm, Log10() for the base-10 logarithm, and Log() for the user-defined-base logarithm. If the natural logarithm of the [x] column was used in the model (using TERR's log() function), the calculated column must also use the natural logarithm of the [x] column (using Spotfire's Ln() function).

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