How to do transformations in Stata
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Basic first steps:

- Draw a graph of the data to see how far patterns in data match the
  simplest ideal patterns. Try @graph@ or @dotplot@.

- See what range the data cover. Transformations will have little effect
  if the range is small.

- Think carefully about data sets including zero or negative values.
  Some transformations are not defined mathematically for some values
  (see above), and often they make little or no scientific sense. For
  example, I would never transform temperatures in degrees Celsius or
  Fahrenheit for these reasons (unless to Kelvin).

Standard scores (mean 0 and sd 1) in a new variable are obtained by

    . ^egen stdpopi = std(popi)^

whereas the basic transformations can all be put in new variables by
@generate@:

    . ^gen recener = 1/energy^
    . ^gen logeener = ln(energy)^
    . ^gen l10ener = log10(energy)^
    . ^gen curtener = energy^^(1/3)^
    . ^gen sqrtener = sqrt(energy)^
    . ^gen sqener = energy^^2^

    . ^gen logitp = log(p/(1-p))^          if p is a proportion
    . ^gen logitp = log(p/(100-p))^        if p is a percent
    . ^gen frootp = sqrt(p) - sqrt(1-p)^   if p is a proportion
    . ^gen frootp = sqrt(p) - sqrt(100-p)^ if p is a percent

Note any messages about missing values carefully: unless you had missing
values in the original variable, they indicate an attempt to apply a
transformation when it is not defined. (Do you have zero or negative
values, for example?)

It is not always necessary to create a transformed variable before
working with it. In particular, @graph@ allows the option ^log^ when
producing a histogram and the options ^xlog^ and ^ylog^ when drawing
a (twoway) scatter plot. The last is very useful because the graph is
labelled using the original values, but it does not leave behind a
log-transformed variable in memory.

Also see:
---------

Review of most common transformations                     help @trreview@

Reasons for using transformations                         help @trreason@

Transformations for proportions and percents              help @trpropor@

Psychological comments -- for the puzzled                 help @trpsych@

On-line: @generate@, @egen@, @graph@
