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Home >> SPSS guide index >> log transform

How to do log transformation in SPSS/PASW


 

Remarks on log tranformations

1.Why might you need apply such a transformation?

When your data is positively(right) skewed and you wish to normalize the data in roder to satisfy normality assumption required by some tests/procedures.

2. Log transform can only be applied to numbers above 0. If you have any numbers which are negative or zero, then one accepted procedure is to add to each number in your data a constant number, which is a bit arbritary (say 3) such that all the numbers will then be above 0. And then apply transformation on this series. (In econometrics, instead of adding an arbitrary number, they use a Heckman type procedure.)

3. How to undo log transform.

(i) Suppose data has been transformed using natural logs (LN). To undo this transformation take exponential of data - EXP.

(ii) Suppose data has been transformed using log10. To undo this, take 10 to the power of each number in the data. Eg. Log10(2) = 0.301, and undoing this we have 10 to the power of 0.301 = 2. [Can do this operation on a basic scientific calculator]