Issues with interacting explanatory variables

[Tom Hetherington]

Hi,
We are trying to combine two explanatory variables to see the effect of them both together. The 2 variables on their own have negative coefficients, however combined the coefficient is positive. This seems very strange and wrong, could there be a reason for this?

[Carlo Lazzaro]

Tom:
it is difficult to say without reading what you typed and what Stata gave you back (as per FAQ). Thanks.