Difference between revisions of "Software tutorial/Transformation of data in a linear model"
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- ``lm(100/y ~ x)`` | - ``lm(100/y ~ x)`` | ||
* - Transformation of :math:`x`: **+, -, /, and ^ do not work on the right hand side!** | * - Transformation of :math:`x`: **+, -, /, and ^ do not work on the right hand side!** | ||
- :math:`y= b_0 + b_1 | - :math:`y= b_0 + \dfrac{b_1}{x}` | ||
- ``lm(y ~ | - ``lm(y ~ 1/x)`` *will give an error* | ||
* - Most transformations of :math:`x` must be wrapped in an AsIs ``I()`` operation: | * - Most transformations of :math:`x` must be wrapped in an AsIs ``I()`` operation: | ||
- :math:`y= b_0 + b_1 | - :math:`y= b_0 + \dfrac{b_1}{x}` | ||
- ``lm(y ~ I( | - ``lm(y ~ I(1/x))`` *will work* | ||
* - Another use of the AsIs ``I()`` operation | * - Another use of the AsIs ``I()`` operation | ||
- :math:`y= b_0 + b_1 x^2` | - :math:`y= b_0 + b_1 x^2` |
Revision as of 15:44, 28 February 2013
<rst>
<rst-options: 'toc' = False/>
<rst-options: 'reset-figures' = False/>
This is shown by example for a few different types of transformations:
.. list-table::
:widths: 15 10 30 :header-rows: 1
* - Description - Desired model - Formula function in R * - Standard, univariate model - :math:`y = b_0 + b_1 x` - ``lm(y ~ x)`` * - Force intercept to zero (check the degrees of freedom!) - :math:`y = b_1 x` - ``lm(y ~ x + 0)`` * - Transformation of an :math:`x` - :math:`y = b_0 + b_1\sqrt{x}` - ``lm(y ~ sqrt(x))`` * - Transformation of :math:`y` - :math:`\log(y) = b_0 + b_1 x` - ``lm(log(y) ~ x)`` * - Transformation of :math:`y` - :math:`100/y= b_0 + b_1 x` - ``lm(100/y ~ x)`` * - Transformation of :math:`x`: **+, -, /, and ^ do not work on the right hand side!** - :math:`y= b_0 + \dfrac{b_1}{x}` - ``lm(y ~ 1/x)`` *will give an error* * - Most transformations of :math:`x` must be wrapped in an AsIs ``I()`` operation: - :math:`y= b_0 + \dfrac{b_1}{x}` - ``lm(y ~ I(1/x))`` *will work* * - Another use of the AsIs ``I()`` operation - :math:`y= b_0 + b_1 x^2` - ``lm(y ~ I(x^2))`` * - Another use of the AsIs ``I()`` operation - :math:`y= b_0 + b_1 (x - \bar{x})` - ``lm(y ~ I(x - mean(x)))``
</rst>