Time-span and coverage
(Source: Introduction to A Linguistic Atlas of Early Middle English, ch. 1, http://www.lel.ed.ac.uk/ihd/laeme1/pdf/Introchap1.pdf)
We take the work of the scribe of the second continuation of the Peterborough Chronicle, writing in 1154/5, as our terminus a quo for early Middle English. This text represents the earliest surviving example of truly Middle English language — that is, it reflects how the spoken language of the scribe’s region of origin had developed in the preceding century. After this there is a gap in attestation: there are very few other surviving texts in newly composed English that can be firmly dated earlier than the last quarter of the 12th century, most of these being ‘ca 1200’. Often manuscripts can be dated only from palaeographic evidence and this method is always approximate. Dates judged from script and decoration are usually given as lying somewhere within a quarter-century range. Were it not for the survival of the Peterborough Chronicle second continuation, we could reasonably say that our terminus a quo would be ca 1200. This is relevant to the selection of our terminus ad quem.
The terminus ad quem is less easily determinable — we do not have a last text in the same way as we have a first text. In addition, the existence of LALME creates certain responsibilities: we would like the two atlases to give as coherent a picture as possible of Middle English ‘as a whole’ without excessive duplication. A linguistic atlas displays how different forms of language change through time and how they vary across space. This leads to a further consideration: selection of too broad a time-span for a dialectal survey makes it more difficult to assess the relationship between diachronic change and regional variation. This is because over an extended period, the results of linguistic change are likely to present us with an intractably large typological range. On the other hand, choice of too narrow a time-span incurs the risk that the surviving texts will not provide sufficiently dense geographical coverage to constitute a continuum.
For LALME, a period of 100 years was considered optimal. For LAEME, in practice it is necessary to select a rather longer time-span  to allow maximal regional coverage in a period from which fewer texts survived. In other words, wider temporal extension is a heuristic that increases our chances of encountering written material originating from different regions, which is especially important for early Middle English when the survival of texts is geographically very patchy. Moreover, dependence on data from an accidentally surviving sample means that our optimal time-span is not the same across the country.
For more information, see Introduction to LAEME, chapter 1.
 This greater extent is in part illusory, as the Peterborough Chronicle second continuation in effect occupies a 25-year band on its own, so the 1150–1325 we give as our time-span is not in fact continuous.