In my previous post on the possible airship panic of 1915, I used n-grams for 'Zeppelin' and 'rumour' derived from the British Newspaper Archive to try to work out when exactly it was. The answer was that it began in late December 1914 and finished in early February 1915. But is the answer right? Is the method useful? To test that I switched to close reading and checked whether it does indeed relate to an airship panic. The very preliminary upshot is that the principle seems sound, but its application is difficult, or at least more complicated than I've allowed -- while the signals in the data are more or less real, there are a substantial number of false positives and false negatives. There will always be some of these, due to bad OCR or just chance, but there are a couple of systematic reasons increasing the difficulties.
The first problem, an obvious one, is that a wider range of key terms is needed: 'Zeppelin' is pretty sound for this period (though occasionally 'airship' and 'dirigible' appear in its place), but 'rumour' needs to be supplemented by other terms such as 'panic', 'scare', or even a phrase like 'it is said'. And of course ostensibly sober and objective analysis of the prospects of Zeppelin raids won't get picked up by a search like mine which is looking for discussions of panic. So there are these false negatives (things I've missed) -- such as the claim in early January that Germany was planning to send ten Zeppelins to raid London at the end of the month with 'the task of crippling the British fleet, and also cause a panic', or the story that a 'huge box kite' being used to test London's anti-aircraft defences led to speculation 'as to whether [a?] Zeppelin has arrived at last'.1 The second, and less soluble, problem is that BNA isn't always very good at detecting when one article starts and another one begins. In fact, it's quite common to find a dozen or two short articles joined together. This means that BNA could (say) consider an article mentioning Zeppelin operations over France, and another discussing rumours that the Germans have mined Brussels so that they can destroy the city if forced to evacuate it, to be part of the same article.2 These are false positives.