I agree with Dr. Rose, but we also have to identify the benefits versus the costs. If we're establishing that we're not exactly sure how effective advertising is in the first place, then to what extent do we have to say that you can't have any advertising any time there's a by-election. Something to keep in mind about a by-election is that these are not necessarily events that are within the government's control. Members leave for different reasons. Sometimes somebody might have passed away, for example.
To answer your question about threshold, which I think is a fair one, you may want to think about the number of by-elections that are occurring at the same time or within a given period. If you happen to have a group of by-elections, the media and everybody else tends to pay a lot more attention. If there's just a single by-election, the national coverage of that single by-election is actually quite limited, so you may want to think about the number of by-elections that happen to be occurring.