<p>There were two very similar posts on CV - <a href="https://stats.stackexchange.com/questions/125909/forecast-using-arima-models">this</a> and the <a href="https://stackoverflow.com/questions/27201188/which-code-to-be-used-for-forecasting-arima-model/27201634#27201634">one</a> it (almost) duplicates (at least, <em>if</em> you count the Q in comments, it's a duplicate). I marked the linked one as a duplicate and was in the process of answering the other, but some computer problems delayed my answer, which was then migrated (and my in-progress answer was lost in the migration, but I have since made a rough version of it in the migrated one).</p>

<p>The closed one clearly is on topic here, since it's a question about properties of forecasting time series.</p>

<p>Either one of two situations exist:</p>

<ol>
<li><p>The one I linked above really is a duplicate, in which case the thing it duplicates must also be on topic and so should never have been migrated</p></li>
<li><p>It isn't a duplicate, in which case it should be reopened.</p></li>
</ol>

<p>I believe the first applies (that the question in comments in the original post made it on topic, and at the same time made the second one a duplicate); however, picking either (1) or (2) is reasonable.</p>

<p>In respect of the post I linked above, as it stands currently the post it was closed as a duplicate of isn't a post on CV, so on what grounds is it a duplicate? It can only be a duplicate of the other one if the other one is brought back here.</p>

<p>It's possible it's a duplicate of another post, of course, but as it stands, something is wrong.</p>
