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What is the command for lagging an independent variable that I also take the logarithm on?
For example: If have the variable pbeef, when estimating an equation, do i write log(pbeef(-1)) or how should it be written?

also, when Writing log(pbeef) is natural or base ten logarithms used?

Yes, log(y(-1)) is the correct syntax. In order to see it for yourself, you can always create a separate variable, series ln_y1 = log(y(-1)), and compare it to series ln_y = log(y).

And log denotes the natural logarithm in EViews. You can always use the command window to check or verify such operations: scalar z = log(10) or scalar z = log(exp(1)).

If you do not enjoy looking up help files, Chapter 4 of EViews Illustrated has a very nice discussion on data transformations...