I don't see any necessary problem with asking a question like that. Here are some things to be sure you do:
- Provide a complete and correct citation for the paper.
- Provide a link to the paper.
- Paste into the body of your question the equation / text you need help understanding (i.e., don't just say 'I need help with equation 5').
- Paste in enough of the context that the question can stand on its own and potential answerers don't need to read the paper to answer your question, if they don't want to.
I recognize that the existence of 4 seems inconsistent with 1 and 2, but you should do all four.
N.B., you say that your question is statistical. I have no reason not to take you at your word. But if your question were only about, say, the derivation of the equation, 'how does equation $i$ fall out of equation $i-1$', it may be a better fit on SE's math site. Likewise, if it were about the economic implications of the equation, it may be a better fit on SE's economics site, etc.