Difference between revisions of "Software tutorial/Transformation of data in a linear model"
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Kevin Dunn (talk | contribs) (Created page with "{{Navigation|Book=Software tutorial|previous=Testing a linear model in R|current=Tutorial index|next=Investigating outliers, discrepancies and other influential points}} __NOT...") |
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- ``lm(y ~ x + 0)`` | - ``lm(y ~ x + 0)`` | ||
* - Transformation of an :math:`x` | * - Transformation of an :math:`x` | ||
- :math:`y = b_0 + \sqrt{x}` | - :math:`y = b_0 + b_1\sqrt{x}` | ||
- ``lm(y ~ sqrt(x))`` | - ``lm(y ~ sqrt(x))`` | ||
* - Transformation of :math:`y` | * - Transformation of :math:`y` |
Revision as of 15:42, 28 February 2013
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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 + b_1 \times 20/x` - ``lm(y ~ 20/x)`` *will give an error* * - Most transformations of :math:`x` must be wrapped in an AsIs ``I()`` operation: - :math:`y= b_0 + b_1 \times 20/x` - ``lm(y ~ I(20/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)))``
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