# Taking logarithms of variables [closed]

It is common to taking logs of variables when perform a regression analysis. But the observations will become negative after logs transformation when it is less than 1. Is this the possible concerns? (e.g. log(0.5) = -0.6931, but log(5)= 1.6094)

• Can you use negative temperatures in a weather forecast or does it break the regression? – Giskard Sep 6 '16 at 13:42
• If your variables of concern are always positive, and your regression extrapolates to give a negative prediction, you could have problems. But if you have taken logarithms and then get a negative prediction then take anti-logarithms (exponentiation if working in base $e$) of that to give you a positive result – Henry Sep 6 '16 at 13:50
• @Henry I do not understand. Why would it be a concern that the predicted value of log of a variable is negative? This does not make the variable itself negative. – Giskard Sep 6 '16 at 14:22
• @denesp: exactly the opposite. If you are modelling, for example, hours worked, and your extrapolation gives a negative result for certain circumstances, that may be an issue. But if you are modelling logarithms of hours worked and your extrapolation gives a negative result for certain circumstances, that is less of a problem as it translates into a small positive number of hours ($\lt 1$) – Henry Sep 6 '16 at 14:26
• What is the underlying problem you are tring to solve? The question as it stands is barely meaningful. – 410 gone Nov 13 '16 at 16:37

No, $\log(x)$ being negative is not a problem what so ever.